Bug 1062100. Part 2: Assign clip rects to non-async-scrollable scrolled layers, and hide clipped layers if some non-moving layer covers their entire clip rect. r=mattwoodrow

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extra : rebase_source : bdc498a51f323d8536746e5a6faf6094a243a984
Bug 1062100. Part 3: When layers are flattened, let all the layers contribute opaqueness to the container layer, whatever their animated geometry roots are. r=tn

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extra : rebase_source : 255807b178162f62082da0fe2bed2315cf4c0581