The property is deprecated and the current implementation simply
redirects it to ClutterActor::background-color, so remove it.
Also update the tests to set the background color directly.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1332
Port tests to use API such as cogl_framebuffer_push_matrix() instead of
cogl_push_matrix() all over the Clutter tests, with one exception:
cogl_polygon(). It'll be ported over in a separate commit, as it is less
straight forward.
Implicitly set CoglMaterial properties are changed to explicitly created
and destructed CoglPipelines with the equivalent properties set.
cogl_push|pop_framebuffer() is replaced by explicitly passing the right
framebuffer, but tests still rely on cogl_get_draw_framebuffer() to get
the target framebuffer.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/935
This is for all intents and purposes the same as
`cogl_object_ref/unref`, but still refers to handles rather than
objects (while we're trying to get rid of the former) so it's a bit of
unnecessary redundant API.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/451
