gdk: enable rgba visuals by default

Unless GDK_RGBA=0 is set.

https://bugzilla.gnome.org/show_bug.cgi?id=630217

gdk/x11/gdkvisual-x11.c

@@ -285,6 +285,14 @@ _gdk_x11_screen_init_visuals (GdkScreen *screen)
         }
     }
 
+  /* prefer the RGBA visual unless GDK_RGBA=0 */
+  if (x11_screen->rgba_visual)
+    {
+      const gchar *csd_env = g_getenv ("GDK_RGBA");
+      if (g_strcmp0 (csd_env, "0") != 0)
+          x11_screen->system_visual = x11_screen->rgba_visual;
+    }
+
 #ifdef G_ENABLE_DEBUG
   if (_gdk_debug_flags & GDK_DEBUG_MISC)
     {