Code's more readable when SkPMFloat is an Sk4f. #floats BUG=skia: BUG=skia:3592 Committed: https://skia.googlesource.com/skia/+/6b5dab889579f1cc9e1b5278f4ecdc4c63fe78c9 CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Ubuntu-GCC-Arm64-Debug-Android-Trybot Review URL: https://codereview.chromium.org/1061603002