Overlay plane PoC with legacy API

Let's try to demonstrate that we could

• [overlay plane + gbm_surface] Render to an overlay plane (i.e. target an overlay plane with a QWindow instead of the primary plane)
• [overlay plane + gbm_bo/fb handle] Associate overlay planes with a bo (which could have been imported via dmabuf) received from outside the platform plugin
• [primary plane + gbm_bo/fb handle] A bo could also be flipped to the primary plane in place of the front bo of the gbm_surface of the QWindow/QScreen (e.g. useful when wanting to scan out a fullscreen client's buffer as-is in a compositor, skipping all composition)

Note: Could be that taking an fb handle is more reasonable than a gbm_bo (no gbm dependency, would work directly with in-process dumb buffers, etc.)

Ignore issues from non-atomic plane management for now.