While completely untested, at least this makes it work "in theory"
again. Before it'd listen to signals on the stage, but have an incorrect
type signature to handle the test paint procedures, meaning it'd
probably crash or cause memory corruptions.
What was needed was a signal which in the callback the test could call
some cogl functions to paint on the framebuffer. While there is no such
signal on the stage, and the ClutterActor::paint signal (which they
probably used in the past) is long gone, lets add a "test actor" that is
just a wrapper that adds that paint signal with a paint context.
The tests that need it are changed to add this actor to the stage, and
to listen to the paint signal on the actor instead of incorrectly
listening on stage signals.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2807>
64c3b824fa