cursor-renderer: Add a cursor painted signal

This signal allows interested parties to be notified of a new cursor
frame being painted regardless of whether it's being painted by the
backend directly or if it's a software rendered cursor frame handled
by clutter.

https://bugzilla.gnome.org/show_bug.cgi?id=749913

src/backends/meta-cursor-renderer.h

@@ -78,4 +78,7 @@ void meta_cursor_renderer_realize_cursor_from_xcursor (MetaCursorRenderer *rende
                                                        MetaCursorSprite   *cursor_sprite,
                                                        XcursorImage       *xc_image);
 
+void meta_cursor_renderer_emit_painted (MetaCursorRenderer *renderer,
+                                        MetaCursorSprite   *cursor_sprite);
+
 #endif /* META_CURSOR_RENDERER_H */