Regression testing

As per https://bugs.freedesktop.org/show_bug.cgi?id=60383 and http://markmail.org/message/qfeknjap5sypisuy#query:+page:1+mid:cv666e3727vvvhz3+state:results

It would be good to create a regression testing suite which ensures that performance doesn't reduce and that known results don't differ compared to known correct outputs.

This requires:
- List of previously defective or good complex PDFs
- Render them with all available backend engines
- Keep track of how long it took to render the files
- Visually compare the results to previous results
- Warn if images are over a significant threshold
- Graph performance records to see performance

This could also be used to provide a fairly comprehensive unit test with code coverage.