get rid of troublesome Op::round

We really only need to_unorm(),
and that's fine with trunc(mad(x, scale, 0.5)).

Change-Id: I1561c678501963a9ae53c22994fc906159fc7199
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/275075
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
5 files changed