Extend the existing output-damage test to test
blit_shadow_to_output() specifically. This function had problems
originally, so make sure they can't reappear.
The added quirk is explained in the test.
An additional check of the quirk in gl_renderer_output_create() ensures
that the shadow framebuffer is really used. The test could false-pass if
the shadow is not used.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>