compositor: add support for underlays

There are many, many situations, in which we can't put surfaces on an overlay, for example

• the window has server side decorations with rounded corners
• there's subtitles on top of it
• there's an entire window on top of it

These situations can still work however, often even more efficiently, by putting the overlay plane below the primary plane and painting a transparent hole on the primary plane where the surface would normally be. Other elements, like window outlines, other windows and subsurfaces are then painted on top of the transparent hole, and are thus still visible.

Note that in its current state, this doesn't work on Intel, as it has fixed plane positions. Fixing that will require putting some more changes on top of this, to put the scene on an overlay plane and the underlay on the primary one.