Factor out meta_actor_painting_untransformed()

The painting_untransformed() function in MetaWindowGroup is useful
elsewhere, in particular if we want to check whether we can avoid
bilinear filtering when painting a texture 1:1.

https://bugzilla.gnome.org/show_bug.cgi?id=735632

src/compositor/clutter-utils.h

@@ -31,4 +31,9 @@ gboolean meta_actor_is_untransformed (ClutterActor *actor,
                                       int          *x_origin,
                                       int          *y_origin);
 
+gboolean meta_actor_painting_untransformed (int         paint_width,
+                                            int         paint_height,
+                                            int        *x_origin,
+                                            int        *y_origin);
+
 #endif /* __META_CLUTTER_UTILS_H__ */