Clients using EGLStream-backed buffers will expect the stream to be
functional after wl_surface::attach(). That means the compositor-side
stream must be created and a consumer attached to it.
To resolve the above, this change realizes buffers even when the attach
operation is deferred (e.g. synchronized subsurfaces).
https://bugzilla.gnome.org/show_bug.cgi?id=782575
(cherry picked from commit 22723ca371)
79b3f56bd3