Bug 1067748. When a solid-color display item covers everything else in its layer, make it a ColorLayer. r=tn

--HG--
extra : rebase_source : e0fdaddfd24c59531ffef865b46f948ba203acaa
Bug 1051636. Don't mark video ImageLayers as opaque. r=tn
The visible region of the layer is not necessarily restricted to the image
that is will be rendered.

Also, we don't know for sure an opaque video frame is going to appear.

--HG--
extra : rebase_source : c6e0f1f0a49b5722b917718634439f045e8a535d