meta-window-actor: Fix paint_volume

We cannot intersect the the complete volume with the unobscured bounds
because it does not include the shadows. So just intersect it with the
windows's shape bounds and union it with the shadow bounds.

This also matches what the comment in the code says:
"We could compute an full clip region as we do for the window texture,
but the shadow is relatively cheap to draw, and a little more complex to clip,
so we just catch the case where the shadow is completely obscured
and doesn't need to be drawn at all."