Move float<->byte conversions into Sk4f.

This lets us avoid conversions to [0.0, 1.0] space and rounding that aren't necessary
for SkColorCubeFilter_opts.h.

Dropping rounding on the way back to bytes means we'll see a bunch of off-by-1 diffs.

Rough perf effect:
  SSSE3: 110 -> 93  (~15%)
  NEON: 465 -> 375  (~20%)

This is the beginning of the end for SkPMFloat as an entity distinct from Sk4f.
I've kept it for now so I can convert sites one by one and think about how things
that really want to keep PM color order will work.

BUG=skia:4117

Review URL: https://codereview.chromium.org/1319413003

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