gl-renderer: Make dummy surface current after all outputs are gone

When all outputs are gone, there are no current read/write
surfaces associated with a context. This makes the previously
created dummy surface current until an output gets attached
to avoid any potential crashes.

v2:

- Remove unnecessary objects

Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>