This makes the test suite run the subsurface-shot with GL-renderer as well.
This is the first time we get GL-renderer exercised in the test suite and in CI at all. In CI it will run with llvmpipe (whatever the default software renderer is), and if you run the test suite on your machine, it uses your system setup (e.g. a hardware driver).
Because GL-renderer produces slightly different pixels from the Pixman-renderer, this MR adds fuzzy pixel matching support in the test helpers. Pixman-renderer is still considered the reference renderer.
Fixes #310 (closed).
Edit: issue #358 (closed) was added to actually enable this testing in CI, since we cannot yet run it with Mesa llvmpipe. The tests will run on GL-renderer by default, but CI configures the build to skip them. Hardware drivers should be fine.