window-actor/x11: Fix using shape region for input

There were two issues with using the shape region to derive an input region.

Firstly, the shape region is against the client rectangle, while the surface actor needs it to be against the buffer rectangle. Fix this by offsetting the shape region before passing it along.

Secondly, we can't just intersect the shape and input region, since that leaves out the window decorations. Fix this by only intersecting the input region covering the client part, and the shape region, and then union that with the input region covering the rest.