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] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #10881 (github issue number if applicable).

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- [x] There are tests for these changes OR
- [x] These changes do not require tests because _____

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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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tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini

@@ -1,4 +0,0 @@
-[transform-input-017.htm]
-  type: reftest
-  expected: FAIL
-  bug: https://github.com/servo/servo/issues/10881

tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini

@@ -1,4 +0,0 @@
-[transform-input-018.htm]
-  type: reftest
-  expected: FAIL
-  bug: https://github.com/servo/servo/issues/10881

tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-019.htm.ini

@@ -1,4 +0,0 @@
-[transform-input-019.htm]
-  type: reftest
-  expected:
-    if os == "linux": FAIL