add pixel snapping conditional on presence of raster cache (#35981)

[jonahwilliams]

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[flutter-dashboard]

This pull request was opened against a branch other than main. Since Flutter pull requests should not normally be opened against branches other than main, I have changed the base to main. If this was intended, you may modify the base back to flutter-3.2-candidate.5. See the Release Process for information about how other branches get updated.

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[skia-gold]

Gold has detected about 1 new digest(s) on patchset 2.
View them at https://flutter-engine-gold.skia.org/cl/github/36022