window-actor: Fix the input shape region for decorated X11 windows

The code here before was completely wrong. Not only did it mix up
coordinate spaces of "client rect" vs. "frame rect", but it used
meta_frame_get_frame_bounds, which is specifically for the *visible*
bounds of a window!

In the case that we don't have a bounding or input shape region at
all on the client window, the input shape that we should apply is
the surface's natural shape. So, set the region to NULL to get the
natural rect picking semantics.