scene/workspacescene: do occlusion testing for direct scanout

Some clients have two or more completely opaque surfaces stacked on top of each other, optimizing the lower ones out makes direct scanout happen more often and more efficiently when multiple planes are involved

In my testing Firefox still doesn't get direct scanout with fullscreen video playback and a mismatching aspect ratio, but it also has really weird opaque regions and a fullscreen black shm buffer instead of 1x1 with viewporter, so that's a client problem...