add gamma stage

Until now we've been using 3 separate parametric stages to apply
gamma to r,g,b.  That works fine, but is kind of unnecessarily
slow, and again less clear in a stack trace than seeing "gamma".

The new bench runs in about 60% of the time the old one does
on my Trashcan.

BUG=skia:6939

Change-Id: I079698d3009b081f1c23a2e27fc26e373b439610
Reviewed-on: https://skia-review.googlesource.com/32721
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

0 files changed, 0 insertions, 0 deletions