Rename SkSourceGammaTreatment to SkDestinationSurfaceColorMode

This is much more explicit about what that type represents (are we in
legacy mode or not), which also makes it suitable for other (upcoming)
usage.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4529

Change-Id: Iacb397c34e7765f1ca86c0195bc622b2be4d9acf
Reviewed-on: https://skia-review.googlesource.com/4529
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
52 files changed