The DBus listener naively create, update and destroy textures without
taking into account other listeners. The texture were shared, but
texture update was unnecessarily duplicated.

Teach DisplayGLCtx to do optionally shared texture handling. This is
only implemented for DBus display at this point, however the same
infrastructure could potentially be used for other future combinations.

Reported-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>