rename gamma stage gamma_

In some setups we see StockStage::gamma somehow
shadowing `double gamma(double)` from math.h.

No big deal.  We do the same for plus_ and xor_.

Change-Id: Ib5d45edf33dcfbc1d0eef1806674fa908e752200
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209788
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed