clutter/backend: Introduce a fallback scale set to primary monitor scale

Add private API to ClutterBackend to set a fallback resource scale
available to Clutter. This API will be used for "guessing" the
resource-scale of ClutterActors in case the actor is not attached to a
stage or not properly positioned yet.

We set this value from inside mutters MetaRenderer while creating new
stage-views for each logical monitor. This makes it possible to set the
fallback scale to the scale of the primary monitor, which is the monitor
where most ClutterActors are going to be positioned.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1276

clutter/clutter/clutter-backend-private.h

@@ -53,6 +53,8 @@ struct _ClutterBackend
   gfloat units_per_em;
   gint32 units_serial;
 
+  float fallback_resource_scale;
+
   ClutterStageWindow *stage_window;
 
   ClutterInputMethod *input_method;
@@ -134,6 +136,12 @@ void                    clutter_set_allowed_drivers                     (const c
 CLUTTER_EXPORT
 ClutterStageWindow *    clutter_backend_get_stage_window                (ClutterBackend         *backend);
 
+CLUTTER_EXPORT
+void clutter_backend_set_fallback_resource_scale (ClutterBackend *backend,
+                                                  float           fallback_resource_scale);
+
+float clutter_backend_get_fallback_resource_scale (ClutterBackend *backend);
+
 G_END_DECLS
 
 #endif /* __CLUTTER_BACKEND_PRIVATE_H__ */

clutter/clutter/clutter-backend.c

@@ -1031,3 +1031,16 @@ clutter_backend_get_default_seat (ClutterBackend *backend)
 
   return CLUTTER_BACKEND_GET_CLASS (backend)->get_default_seat (backend);
 }
+
+void
+clutter_backend_set_fallback_resource_scale (ClutterBackend *backend,
+                                             float           fallback_resource_scale)
+{
+  backend->fallback_resource_scale = fallback_resource_scale;
+}
+
+float
+clutter_backend_get_fallback_resource_scale (ClutterBackend *backend)
+{
+  return backend->fallback_resource_scale;
+}

src/backends/meta-renderer.c

@@ -153,6 +153,19 @@ meta_renderer_real_rebuild_views (MetaRenderer *renderer)
     {
       MetaLogicalMonitor *logical_monitor = l->data;
 
+      if (meta_logical_monitor_is_primary (logical_monitor))
+        {
+          ClutterBackend *clutter_backend;
+          float scale;
+
+          clutter_backend = meta_backend_get_clutter_backend (backend);
+          scale = meta_is_stage_views_scaled ()
+            ? meta_logical_monitor_get_scale (logical_monitor)
+            : 1.f;
+
+          clutter_backend_set_fallback_resource_scale (clutter_backend, scale);
+        }
+
       meta_logical_monitor_foreach_crtc (logical_monitor,
                                          create_crtc_view,
                                          renderer);