Culling fixes

This fixes two bugs:

• windows with XShape set were culled out incorrectly (broken by me in !1058 (merged))
• subsurfaces were not culled out any more (broken by me in !1049 (merged))

And, as a new features, enables culling of Wayland surfaces without opaque region that do not have an alpha channel and saves some allocations.

Fixes #1061 (closed)