[RFC] Add backend capability bitmask
Created by: ascent12
This allows for a more robust an extensible way to advertise features that a backend is capable of, instead of just trying a function and seeing if it fails. I've just added the enum here; I thought I would get some feedback before actually bothering to plumb this up.
This is something I want for the renderer PR and potential future features we'd want to add. It may be worth adding a separate bitmask for wlr_outputs too.