gfx: Switch the default to CPU painting.

We've discussed this some and I think there's consensus to do it as a
pragmatic decision for now. CPU painting is more stable, especially with
buggy drivers, and faster (because we aren't caching the necessary
OpenGL objects yet and possibly for other reasons), so it provides a
better "out of the box" experience for newcomers to Servo who don't know
to pass the `-c` option. This patch continues to reftest both Skia and
Skia-GL out of a desire to keep options open. Skia-GL remains a
first-class citizen.

r? @metajack