Bug 841601 - Add background-blend-mode to the style parsing mechanism. r=heycam
Bug 841601 - Add background-blend-mode implementation. r=roc
Bug 841601 - Add background-blend-mode tests. r=roc
Bug 841601 - Add a second batch of tests for background-blending which tests each blend mode. r=roc

The reference results are copied over from the svg tests.