Preferred buffer format/modifier feedback loop

[emersion]

It's still premature to plan anything, but in the long run a goal would be to indicate the preferred buffer/modifier format to clients.

The idea would be to figure out whether a client could be put into a plane, if so send some hint to suggest using a buffer compatible with the plane's IN_FORMATS. Then hope for the client to change its buffer format and hope that the buffer can be scanned out.

Wayland protocol draft for this feature: https://patchwork.freedesktop.org/series/52370/