Add test for 5121: Blending simple elements
Add test for 5122: Blending video with sibling. Remove older test: blending_video.
Add test for 5123: Blending with overlapping sibling. Rework older test: mix-blend-mode-blending-with-sibling-ref.html.
Add test for 5124: Blending element is interposed. Rework older test: mix-blend-mode-blending-two-levels-of-ascendants.html.
Updates
Remove old code
Add text/css for styles
Address review comments
Update video test ref title
Address mibalan's review comments: add meta flags
Merge pull request #409 from mihaitica/mbm_s2

Rework the second section of the mix-blend-mode test plan