ClutterTexture has many features that we simply don't use and don't make
sense for a subclass with custom drawing. Deriving directly from ClutterActor
simplifies our code by avoiding workarounds and makes things more robust.
Additionally, make it public. GNOME Shell was already assuming that any
MetaShapedTexture was also a ClutterTexture, and we need to replace these
bits with new API for GNOME Shell to use.
https://bugzilla.gnome.org/show_bug.cgi?id=660941
