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
actor_is_untransformed is a function meta-window-group uses to determine
if an actor is relatively pixel aligned and not contorted. It then
returns the coordinates of the actor.
In a subsequent commit will need the function in a different file, so
this commit separates it out.
https://bugzilla.gnome.org/show_bug.cgi?id=682427
