use round() instead of trunc() to f32->unorm

This does open us up to a little bit of possible inconsistency of
rounding when right on a x.5 (sometimes we'll +0.5 and trunc, sometimes
round to nearest, sometimes round according to the default mode which is
usually round to nearest) but I think that inconsistency may be worth
the free register not needing a splat(0.5f) buys us.

A few invisible diffs.

Change-Id: I9af092c937ccf7c5891c2ab3cb298d217e4a9e9f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253725
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>

Author: Mike Klein