Do not paint windows behind an opaque window
For performance reasons, any windows behind an opaque one should not be painted at all. This should greatly improved performances when using for example the max layout of Awesome.
If a window is opaque, avoid painting occluded area (refs #22).
This is a naive implementation because the background is still painted
needlessly and this function should not even be called if the window is
completely occluded (but that would require managing regions on the