Bug 961249 - Don't propagate component alpha layer flattening across force active layers and avoid unnecessary invalidations. r=roc

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extra : rebase_source : 60a08e1351c5ce1aa4ced54e7ef2fc18cb0e7590
Bug 1041893 - Stop tracking the clip direction in GetBackgroundClip since it has no effect. r=roc

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extra : rebase_source : 3d54b72eeabcb90aaf0179e7a382406facff6b5f