Auto merge of #12742 - mrobinson:off-by-one-ng-2, r=pcwalton

Simplify and extend trans-stacking-context subpixel accumulation

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- [x] These changes fix #10881 (github issue number if applicable).

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Simplify the situations in which subpixels are accumulated, so that it
is only done for translation or identity transformation matrices. Also,
apply accumulated subpixels to more operations in PaintContext. This
fixes several pre-existing reftests and hopefully will eliminate
off-by-one errors in flaky reftests.

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