The testing currently done is:

 * Creating a virtual monitor succeeds and gets the right configuration
 * Painting a few times results in the expected output
 * Changing the content of the stage also changes the painted content
   accordingly
 * Destroying the virtual monitor works as expected

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1698>

This adds a test framework that makes it possible to compare the result
of painting a view against a reference image. Test reference as PNG
images are stored in src/tests/ref-tests/.

Reference images needs to be created for testing to be able to succeed.
Adding a test reference image is done using the
`MUTTER_REF_TEST_UPDATE` environment variable. See meta-ref-test.c for
details.

The image comparison code is largely based on the reference image test
framework in weston; see meta-ref-test.c for details.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1698>