tests: extend output-damage to GL shadow framebuffer

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>

include/libweston/libweston.h

@@ -196,6 +196,8 @@ enum weston_hdcp_protection {
 struct weston_testsuite_quirks {
 	/** Force GL-renderer to do a full upload of wl_shm buffers. */
 	bool gl_force_full_upload;
+	/** Ensure GL shadow fb is used, and always repaint it fully. */
+	bool gl_force_full_redraw_of_shadow_fb;
 };
 
 /** Weston test suite data that is given to compositor