Bug 969354. Track which ThebesLayers contain items which returned true from ShouldFixToViewport, and make sure each such item gets its own ThebesLayer. r=mattwoodrow

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extra : rebase_source : 629f91869bd4c4ad2e4e17c40c5fe0311532fb67
Bug 973767. Put the compositor into test mode via an IPDL transaction. r=mattwoodrow

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extra : rebase_source : d40cf8624f93b85298cc501ff315522220522fb6