Draft: wayland: Add single-pixel-buffer offload optimization

This is a simple PoC for replacing the buffer of the main surface with a black single-pixel-buffer when the window is fullscreen and a subsurface is on top, allowing video players like the demo-player or livi to signal Wayland compositors that they only need a single hardware plane for KMS offloading.

It does not yet have a render-side check that the main-surface content is actually all black and also attaches the buffer on every frame.