less naive lowp constant_color

This is as good as we can get without switching away from float inputs.

All diffs due to rounding (from the +256.0f).

Change-Id: I0d314f111d313577ce9078660178be17e865f11e
Reviewed-on: https://skia-review.googlesource.com/18845
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
3 files changed