Change SkUnitScalarClampToByte to more accurate implementation.

The previous implementation was likely more efficient when SkScalar was SkFixed.

BUG=skia:4632
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1691403002

Review URL: https://codereview.chromium.org/1691403002
1 file changed