This is enough for testing that surface.cairo is a cairo Surface.
Use cairocffi when available.
Make the test suite 5x faster.

When comparing image pixels:

* Inline the get_pixel lambda
* Move it out of the loop
* Compare big chuncks (whole image, row, RGBA pixel) to avoid entering
inner loops if they are equal.

Needs review for correctness.
Merge branch 'tests-speed' into cffi
Use cairo instead of pysticia to load PNG test results.
Merge branch 'master' into cffi
Merge branch 'master' into cffi