Port HiDPI support from slick-greeter.

 Inspired by these slick-greeter commits:

 commit ffd43c4d425e70bc003b490dcddb8fbae1fffea7
 Author: Clement Lefebvre <clement.lefebvre@linuxmint.com>
 Date:   Sat Apr 8 14:13:23 2017 +0100

    HiDPI: Allow to force HiDPI support ON or OFF

 commit 809f1b30a321ae12fdd3ba154791f9ead291acc7
 Author: Clement Lefebvre <clement.lefebvre@linuxmint.com>
 Date:   Fri Apr 7 23:04:50 2017 +0100

    HiDPI: Take the screen size in consideration

    HiDPI shouldn't be activated on large screens where the dpi is OK.

 commit 851fca4e5508471d6f388edcd0327c3fc5b2b12f
 Author: Clement Lefebvre <clement.lefebvre@linuxmint.com>
 Date:   Fri Apr 7 18:43:42 2017 +0100

    Add HiDPI support

    Only support 1x and 2x scale ratios for now.

    Also add a setting so the user can disable HiDPI support.

6 files changed, 107 insertions, 3 deletions

diff --git a/Makefile.am b/Makefile.am
index 7ba4009..730cceb 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -10,6 +10,7 @@ sbin_SCRIPTS = arctica-greeter-guest-account-script
 pkglibexec_SCRIPTS = lightdm-arctica-greeter-session \
 	arctica-greeter-guest-session-auto.sh \
 	arctica-greeter-guest-session-setup.sh \
+	arctica-greeter-check-hidpi \
 	arctica-greeter-set-keyboard-layout
 
 EXTRA_DIST = \
diff --git a/arctica-greeter-check-hidpi b/arctica-greeter-check-hidpi
new file mode 100755
index 0000000..68f0baf
--- /dev/null
+++ b/arctica-greeter-check-hidpi
@@ -0,0 +1,67 @@
+#!/usr/bin/python3
+# -*- coding: utf-8 -*-
+
+# Copyright (C) 2017 Clement Lefebvre <clement.lefebvre@linuxmint.com>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 3 as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# Authors: Clement Lefebvre <clement.lefebvre@linuxmint.com>
+
+import gi
+gi.require_version('Gdk', '3.0')
+from gi.repository import Gdk
+import sys
+import os
+
+HIDPI_LIMIT = 192
+
+def get_window_scale():
+    window_scale = 1
+    try:
+        display = Gdk.Display.get_default()
+        screen = display.get_default_screen()
+        primary = screen.get_primary_monitor()
+
+        rect = screen.get_monitor_geometry(primary)
+        width_mm = screen.get_monitor_width_mm(primary)
+        height_mm = screen.get_monitor_height_mm(primary)
+        monitor_scale = screen.get_monitor_scale_factor(primary)
+
+        # Return 1 if the screen size isn't available (some TVs report their aspect ratio instead ... 16/9 or 16/10)
+        if ((width_mm == 160 and height_mm == 90) \
+            or (width_mm == 160 and height_mm == 100) \
+            or (width_mm == 16 and height_mm == 9) \
+            or (width_mm == 16 and height_mm == 10)):
+            return 1
+
+        if rect.height < 1500:
+            return 1
+
+        if width_mm > 0 and height_mm > 0:
+            witdh_inch = width_mm / 25.4
+            height_inch = height_mm / 25.4
+            dpi_x = rect.width * monitor_scale / witdh_inch
+            dpi_y = rect.height * monitor_scale / height_inch
+            if dpi_x > HIDPI_LIMIT and dpi_y > HIDPI_LIMIT:
+                window_scale = 2
+
+    except Exception as detail:
+        syslog.syslog("Error while detecting hidpi mode: %s" % detail)
+
+    return window_scale
+
+if __name__ == '__main__':
+    window_scale = get_window_scale();
+    print ("{script}: Window scale is {value}".format(script=os.path.basename(sys.argv[0]), value=window_scale), file=sys.stderr)
+    print (window_scale, file=sys.stdout)
+    sys.exit(0)
diff --git a/data/org.ArcticaProject.arctica-greeter.gschema.xml b/data/org.ArcticaProject.arctica-greeter.gschema.xml
index 7e8adc8..95ab139 100644
--- a/data/org.ArcticaProject.arctica-greeter.gschema.xml
+++ b/data/org.ArcticaProject.arctica-greeter.gschema.xml
@@ -106,6 +106,15 @@
       <default>300</default>
       <summary>Number of seconds of inactivity before blanking the screen. Set to 0 to never timeout.</summary>
     </key>
+    <key name="enable-hidpi" type="s">
+      <choices>
+        <choice value='on'/>
+        <choice value='off'/>
+        <choice value='auto'/>
+      </choices>
+      <default>'auto'</default>
+      <summary>Whether to enable HiDPI support</summary>
+    </key>
     <key name="remote-service-fqdn" type="s">
       <default>'service.arctica-project.org'</default>
       <summary>Default FQDN for host offering Remote Logon Service</summary>
diff --git a/debian/arctica-greeter.install b/debian/arctica-greeter.install
index a6be53b..6a5bb10 100644
--- a/debian/arctica-greeter.install
+++ b/debian/arctica-greeter.install
@@ -10,5 +10,6 @@ usr/share/man/
 usr/share/sounds/
 usr/share/xgreeters/
 usr/lib/*/arctica-greeter/lightdm-arctica-greeter-session
+usr/lib/*/arctica-greeter/arctica-greeter-check-hidpi
 usr/lib/*/arctica-greeter/arctica-greeter-set-keyboard-layout
 var/lib/polkit-1/localauthority/10-vendor.d/arctica-greeter.pkla
diff --git a/src/arctica-greeter.vala b/src/arctica-greeter.vala
index adeb480..74be36a 100644
--- a/src/arctica-greeter.vala
+++ b/src/arctica-greeter.vala
@@ -606,6 +606,22 @@ public class ArcticaGreeter
         }
     }
 
+    private static void check_hidpi ()
+    {
+        try {
+            string output;
+            Process.spawn_command_line_sync(Path.build_filename (Config.PKGLIBEXECDIR, "arctica-greeter-check-hidpi"), out output, null, null);
+            output = output.strip();
+            if (output == "2") {
+                debug ("Activating HiDPI (2x scale ratio)");
+                GLib.Environment.set_variable ("GDK_SCALE", "2", true);
+            }
+        }
+        catch (Error e){
+            warning ("Error while setting HiDPI support: %s", e.message);
+        }
+    }
+
     public static int main (string[] args)
     {
         /* Protect memory from being paged to disk, as we deal with passwords */
@@ -629,6 +645,18 @@ public class ArcticaGreeter
         */
         GLib.Environment.set_variable ("GDK_CORE_DEVICE_EVENTS", "1", true);
 
+        log_timer = new Timer ();
+        Log.set_default_handler (log_cb);
+
+        var hidpi = AGSettings.get_string (AGSettings.KEY_ENABLE_HIDPI);
+        debug ("HiDPI support: %s", hidpi);
+        if (hidpi == "auto") {
+            check_hidpi ();
+        }
+        else if (hidpi == "on") {
+            GLib.Environment.set_variable ("GDK_SCALE", "2", true);
+        }
+
         bool do_show_version = false;
         bool do_test_mode = false;
         OptionEntry versionOption = { "version", 'v', 0, OptionArg.NONE, ref do_show_version,
@@ -710,9 +738,6 @@ public class ArcticaGreeter
         Gtk.init (ref args);
         Ido.init ();
 
-        log_timer = new Timer ();
-        Log.set_default_handler (log_cb);
-
         debug ("Starting arctica-greeter %s UID=%d LANG=%s", Config.VERSION, (int) Posix.getuid (), Environment.get_variable ("LANG"));
 
         /* Set the cursor to not be the crap default */
diff --git a/src/settings.vala b/src/settings.vala
index e55b0c2..441035b 100644
--- a/src/settings.vala
+++ b/src/settings.vala
@@ -47,6 +47,7 @@ public class AGSettings
     public const string KEY_REMOTE_SERVICE_FQDN = "remote-service-fqdn";
     public const string KEY_TOGGLEBOX_FONT_FGCOLOR = "togglebox-font-fgcolor";
     public const string KEY_TOGGLEBOX_BUTTON_BGCOLOR = "togglebox-button-bgcolor";
+    public const string KEY_ENABLE_HIDPI = "enable-hidpi";
 
     public static bool get_boolean (string key)
     {