Proposal to change subsurface extension

[John Kåre Alsaker]

I don't see a way to have a group of surfaces in sync with a video
surface operating independently on top work in the current
implementation without locking. It seems that only work for a single
surface and not a group of surfaces. The current implementation then
also requires the ability to map a surface with no content. If we
can't avoid doing that, there's not much reason to not change the
current implementation to a cleaner scheme.

If we absolutely can't add the wl_egl_window_* functions, an
alternative in my proposal would be making wl_surface.commit
configurable (sounds familiar?). It would either commit itself and all
children recursively or do nothing depending on what was configured.
This allows eglSwapBuffers to work, but I don't find it very clean.