add clamp_gamut

This should be cheaper in both float and lowp modes.

Change-Id: If6bba3abd31990b622f010c8fcdebff3288c9d70
Reviewed-on: https://skia-review.googlesource.com/152581
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed