Add tests for the sections 4.1.5 and 4.1.6 from the blending test plan
Merge pull request #395 from Mire/mbudaes/mix-blend-mode_video_tests

Add tests for the sections 4.1.5 and 4.1.6 from the blending test plan: http://test.csswg.org/source/test-plans/compositing-1/test-plan.html