This contains four tests :-
- A regular onscreen source with a clone next to it
- An offscreen source with a clone. This is currently commented out
because it no longer works.
- An onscreen source with a rectangular clip and a clone.
- An onscreen source with a clip from a path and a clone.
The sources are all a 2x2 grid of colors. Each clone is tested that it
either contains the color that should be at that grid position or that
the stage color is showing through if the source is clipped.