wlr_scene: Only consider visible parts of the node when culling background

Originally, I thought that we could safely subtract opaque regions
from the background even if the black rect optimization was kicking in.
This is wrong because a scene node that isn't fully occluded will still
appear in the render list even if its partially under a black rect. We
need to make sure that while culling the background, we only consider
opaque regions that are also visible. This will fix the black rect
optimization with the background.

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