gl: Rollback behaviour change in GstGLDummyWindow
Making it advertised itself as "has_output_surface" as this is the assumption the code was making before that vmethod was added.
Add a subclass of GstGLWindow that is always used for the new
GstGLDisplayEGLDevice display type instead of requiring a
GstGLDummyWindow to be used in that case and this subclass marks
itself as not having output surface. Technically we could have
returned NULL
in GstGLDisplay::create_window
vmethod, but all
the code makes the assumption this vmethod returns something.