arb_framebuffer_srgb: Use a triangle for the fast clear blending test.

These two patches improve the test coverage of doing a blending on a fast cleared surface with combinations of sRGB enabled/disabled during the fast clear, and also during the rendering/blending.

The main change is do a blend in a diagonal of the framebuffer so we can see how the hardware behaves when the blending is not perfectly aligned with the CCS blocks.