When we're unredirected, we don't have a pixmap, and thus our allocation
becomes 0x0. So when events come in, they pass right through our actor,
going to the one underneath in the stack.
Fix this by having a fallback size on the shaped texture actor when
we're unredirected, causing it to always have a valid allocation.
This fixes clicking on stuff in sloppy / mouse mode focus.
We want to remove a bunch of auxilliary duties from the MetaWindowActor
and MetaSurfaceActor, including some details of how culling is done.
Move the unobscured region culling code to the MetaShapedTexture, which
helps the actor become "more independent".
https://bugzilla.gnome.org/show_bug.cgi?id=720631
