For X11 garbage we have to pass some annoying parameters to EGL context
creation. Add some sort of extensible API, so that adding a new
parameter doesn't break all callers. We still want to keep it as a
single function, because it's so nice isolating all the EGL nonsense API
boilerplate like this. (Did I mention yet that X11 and EGL are garbage?)
Also somewhat simplifies the vo_flags mess in the helper internals.