window-actor: Punt mask generation to MetaWindowActor

This effectively makes MetaShapedTexture not a MetaShapedTexture, but a simple
and dumb MetaMaskedTexture, with an optimization for clipped regions.

We're doing this as the mask may need to be more complicated than made of
a cairo path -- we eventually want GTK+ to draw the entire frame background,
which we'll then scan.

https://bugzilla.gnome.org/show_bug.cgi?id=676052