RFC: Add gpu protocol
In order to improve support for VFIO use-cases this protocol allows to suggest the compositor to release control of graphics devices or switch compositing to another device.
The compositing flags are divided in current and preferred because switching on the fly is not currently trivial to do and it might still take a while for that to change. With the preferred flag the intention is that KWin can then save the GPU as being preferred and switch to it when a new session is started.