Better (?) interface for controlling sRGB-ness of mipmaps on GrTexture

Switch to new SkSourceGammaTreatment type. Move to a separate setter,
rather than an optional argument to dirty. Set the flag correctly after
uploading CPU-built mips.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2058143002

Review-Url: https://codereview.chromium.org/2058143002
5 files changed