Run subsurface-shot test with GL-renderer

While developing !287 (merged) I noticed that the subsurface-shot test does not pass if trivially ran with GL-renderer. That is because the background color is defined in floats. That is converted to uint8 by truncation in Pixman-renderer and by rounding in GL-renderer. This causes the reference image created with Pixman-renderer to differ from what GL-renderer produces.

If the theory holds, we need to allow a +1 fuzz in the color values when comparing to reference. That should work when the reference image and the test image have the same bit-depth regardless of what it is.

Implement the fuzz and make subsurface-shot run on both Pixman- and GL-renderers.