make SkGammas less bad

Data's existing operator== is terribly broken.
Instead replace it with what everyone really
wants to know, allChannelsSame().

I'm not quite ready to get rid of SkGammas yet,
but we're getting close.

Bug: chromium:799834

Change-Id: Ibcfd252e380b15fba53eb707f58304a50e90ce9f
Reviewed-on: https://skia-review.googlesource.com/100320
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed