Sharing a GBM won't work on all platforms

Yes, it won't work on NVIDIA. Just like Wayland in general won't work on NVIDIA. For the nested platforms: fine with me, if recording doesn't work. Fbdev is probably something to consider dropping and hwcomposer is also fd based, so we can also pass it around.

and you'll never be able to do partial screen capture on the kwin side.

aye, totally fine on my side. I rather "capture" too many data and let another process do the heavy work to cut it down.

The patch set which is in work is currently lacking a solution for multiple screens. That's kind of the last blocker to integrate it. Unfortunately we are also lacking ideas on how to make it work with multiple screens ;-)