monitor-manager: Remove 'scale' from MetaOutput

Replace the 'scale' of an output with a vfunc on the MetaMonitorManager
class that takes a monitor and a monitor mode which calculates the
scale. On X11 this always returns 1, on KMS, the old formula is used.
On the dummy and test backends, the already configured values are
returned.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/x11/meta-monitor-manager-xrandr.c

@@ -1568,6 +1568,15 @@ meta_monitor_manager_xrandr_is_transform_handled (MetaMonitorManager  *manager,
   return TRUE;
 }
 
+static int
+meta_monitor_manager_xrandr_calculate_monitor_mode_scale (MetaMonitorManager *manager,
+                                                          MetaMonitor        *monitor,
+                                                          MetaMonitorMode    *monitor_mode)
+{
+  /* X11 does not support anything other than scale 1. */
+  return 1;
+}
+
 static void
 meta_monitor_manager_xrandr_init (MetaMonitorManagerXrandr *manager_xrandr)
 {
@@ -1644,6 +1653,7 @@ meta_monitor_manager_xrandr_class_init (MetaMonitorManagerXrandrClass *klass)
   manager_class->tiled_monitor_removed = meta_monitor_manager_xrandr_tiled_monitor_removed;
 #endif
   manager_class->is_transform_handled = meta_monitor_manager_xrandr_is_transform_handled;
+  manager_class->calculate_monitor_mode_scale = meta_monitor_manager_xrandr_calculate_monitor_mode_scale;
 
   quark_meta_monitor_xrandr_data =
     g_quark_from_static_string ("-meta-monitor-xrandr-data");