Unify common backend options
The following discussion from !274 (merged) should be addressed:
Technically, it might be better to make it a tri-state, just like we do for render_type below.
Speaking of which - why is it backend-headless specific? From a quick look we could make it generic alongside other options like
seat(?), etc. Yes some backends like fbdev (another call for removal?) and rdp support only pixman rendering, for those a "not yet supported, patches welcome" should suffice.
Apart from removing code duplication, this will remove confusing cases where generic and backend/module specific configs conflict - one has "use-gl=no" while the other is "use-gl=yes".
See also the reply to the above comment in the original discussion thread.