Bug 1041530. Be more careful about contributing opaque areas to the window region. r=mattwoodrow

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extra : rebase_source : 23f51dfbf716357310ac708a396b325864e41a67
Bug 1042327. Use a more narrow workaround for pixman bug. r=jrmuizel

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extra : rebase_source : 4631ea06598c681a5729e225d4e2f495fbb2622b