Query XSETTINGS for the content scale factor

For GTK-based systems using XSETTINGS it's much more likely to be
available, rather than the "GDK_SCALE" environment variable, which is a
debugging tool according to the GTK documentation.

src/video/x11/SDL_x11modes.c

@@ -23,6 +23,7 @@
 #ifdef SDL_VIDEO_DRIVER_X11
 
 #include "SDL_x11video.h"
+#include "SDL_x11settings.h"
 #include "edid.h"
 
 /* #define X11MODES_DEBUG */
@@ -230,9 +231,13 @@ static float GetGlobalContentScale(SDL_VideoDevice *_this)
             }
         }
 
+        /* If that failed, try the XSETTINGS key... */
+        if (scale_factor <= 0.0) {
+            scale_factor = X11_GetXsettingsIntKey(_this, "Gdk/WindowScalingFactor", -1);
+        }
+
         /* If that failed, try the GDK_SCALE envvar... */
-        if (scale_factor <= 0.0)
-        {
+        if (scale_factor <= 0.0) {
             const char *scale_str = SDL_getenv("GDK_SCALE");
             if (scale_str) {
                 scale_factor = SDL_atoi(scale_str);

src/video/x11/SDL_x11settings.c

@@ -94,4 +94,31 @@ void X11_HandleXsettings(SDL_VideoDevice *_this, const XEvent *xevent)
     }
 }
 
+int X11_GetXsettingsIntKey(SDL_VideoDevice *_this, const char *key, int fallback_value) {
+    SDL_VideoData *data = _this->driverdata;
+    SDLX11_SettingsData *xsettings_data = &data->xsettings_data;
+    XSettingsSetting *setting = NULL;
+    int res = fallback_value;
+
+
+    if (xsettings_data->xsettings) {
+        if (xsettings_client_get_setting(xsettings_data->xsettings, key, &setting) != XSETTINGS_SUCCESS) {
+            goto no_key;
+        }
+
+        if (setting->type != XSETTINGS_TYPE_INT) {
+            goto no_key;
+        }
+
+        res = setting->data.v_int;
+    }
+
+no_key:
+    if (setting) {
+        xsettings_setting_free(setting);
+    }
+
+    return res;
+}
+
 #endif /* SDL_VIDEO_DRIVER_X11 */

src/video/x11/SDL_x11settings.h

@@ -34,5 +34,6 @@ typedef struct X11_SettingsData {
 extern void X11_InitXsettings(SDL_VideoDevice *_this);
 extern void X11_QuitXsettings(SDL_VideoDevice *_this);
 extern void X11_HandleXsettings(SDL_VideoDevice *_this, const XEvent *xevent);
+extern int X11_GetXsettingsIntKey(SDL_VideoDevice *_this, const char *key, int fallback_value);
 
 #endif /* SDL_x11settings_h_ */