Convert SkPMFloat to [0,1] range and prune its API.

Now that Sk4px exists, there's a lot less sense in eeking out every
cycle of speed from SkPMFloat: if we need to go _really_ fast, we
should use Sk4px.  SkPMFloat's going to be used for things that are
already slow: large-range intermediates, divides, sqrts, etc.

A [0,1] range is easier to work with, and can even be faster if we
eliminate enough *255 and *1/255 steps.  This is particularly true
on ARM, where NEON can do the *255 and /255 steps for us while
converting float<->int.

We have lots of experimental SkPMFloat <-> SkPMColor APIs that
I'm now removing.  Of the existing APIs, roundClamp() is the sanest,
so I've kept only that, now called round().  The 4-at-a-time APIs
never panned out, so they're gone.

There will be small diffs on:
colormatrix coloremoji colorfilterimagefilter fadefilter imagefilters_xfermodes imagefilterscropexpand imagefiltersgraph tileimagefilter

BUG=skia:

Review URL: https://codereview.chromium.org/1201343004
9 files changed