Only four drivers actually use the overlay cores: glint, sunffb, chips, and mga.
They're based on cfb, and as a result they don't support Render and have known
correctness and stability bugs.
The solution is to add additional visuals when Composite is active, and force
windows created on those visuals to always be composited even when no compmgr is
active. This would provide a sort of overlay emulation for every driver using
fb, and would allow the removal of the overlay cores.
Sorry about the phenomenal bug spam, guys. Adding xorg-team@ to the QA contact so bugs don't get lost in future.
This actually works fine already for visuals with non-pseudocolor depths. With pseudocolor it's a bit tricky to get the visual and colormap created the right way around.