Ensure only fractional floats are converted to SkFixed in SubpixelAlignment.

BUG=skia:4632
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1779083003

Review URL: https://codereview.chromium.org/1779083003
1 file changed