Test combinations of output and buffer scale and transform

This series improves the screenshot testing features of the test harness and adds two new test programs: output-transforms and buffer-transforms.

The two test programs iterate through a selection of output scale & transform and buffer scale & transform to ensure that all transformations are implemented correctly in both Pixman and GL renderers.

NOTE: When looking at the reference images, keep in mind that while buffer scale is real (the image in the buffer really is scaled up) the buffer transform is a lie. IOW, regardless of the buffer transform, the buffer contents are always oriented the same. Effectively this means that you need to undo the buffer transform to understand what content the compositor sees. This is done to avoid having to write the transform code once more and verifying that that code is correct before it is used to test the compositor.

The commit "tests: add output transform tests" message has more details, including an in-depth explanation on why basic-test-card.png is what it is.

Fixes: #52 (closed)