Migrate the tst suite to using the new screenshooting protocol. This
ensures the new protocol and implementation work, and removes a user of
the old protocol so that the old protocol can be removed in the future.

Now that the compositor chooses the pixel format,
capture_screenshot_of_output() needs to convert to the expected format
in the tests when necessary.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>