Bug 1091296 - Refactor the pre- and post-scale cancellation code into a helper method. r=mattwoodrow

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extra : rebase_source : c06309a16a13673eb59bb98c9dfd4d5c4846aad5
Bug 1091296 - Remove the redundant code for clearing async transforms per frame of composition. r=mattwoodrow

We already reset the shadow transform to the layer transform in
SetShadowProperties() before compositing every frame, so this is redundant.

--HG--
extra : rebase_source : bef51e6a918ffaab134a4b53fb6f2ec6c45d9d6d
Bug 1091296 - When applying the APZ transform on a layer, make sure not to clobber any existing OMTA transform. r=mattwoodrow

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extra : rebase_source : 9662fbdea917d63a1205660979cbb5c6f861ba02