compositor: Don't lift planes out of scene graph entirely

When reviewing !1258 (merged) I thought that some of that patch series could be much simpler if our pnode visibility calculations weren't per-plane.

I threw this together to see what global visibility in pnodes would look like, and am curious if anyone agrees/disagrees with my supposition that it's better to do damage when moving planes than to render damage that's fully occluded beneath them.

I think there's still room for more optimization. Translucent planes need not trigger additional redraw on move - this would clean up the mouse cursor? And we can perhaps take more care not to redraw under the still-occluded parts of opaque regions of planes.

This is not a bug fix, and it may introduce new damage tracking bugs. :)