cursor: Correct switching between fallback and backend cursors

When switching from the stage cursor to the native cursor, we
forgot to repaint the stage to get rid of the old cursor. Fix
this by having the abstract cursor renderer class track whether
we're using the backend, rather than doing chain-up shenanigans.

src/backends/meta-cursor-renderer.h

@@ -51,7 +51,7 @@ struct _MetaCursorRendererClass
 {
   GObjectClass parent_class;
 
-  void (* update_cursor) (MetaCursorRenderer *renderer);
+  gboolean (* update_cursor) (MetaCursorRenderer *renderer);
 };
 
 GType meta_cursor_renderer_get_type (void) G_GNUC_CONST;