arb_sample_shading: Test interaction of apha-to-coverage, sample mask and MRT
requested to merge andrii-krv/piglit:test_for_alpha_to_coverage_with_mrt_and_sample_mask into master
Test verifies correct work of alpha-to-coverage with every variation of sample mask, all levels of MSAA and different number of render targets.
Test currently passes on redeonsi and fails on i965 and iris. mesa!292 (closed) - the fix for i965 and iris.