Refactor parsing and storage of SkGammas

Benefits:
(1) Parses and stores gamma tags in a single allocation.
(2) Recognizes equal gamma tags to skip parsing work and
    save memory.

Non-Benefits:
(1) Not less complicated.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2117773002

Committed: https://skia.googlesource.com/skia/+/2ea944c2b710caf29d4795ac953bad14224796f7
Committed: https://skia.googlesource.com/skia/+/959ccc1f3f49e1ddeb51c32c30ac4a2d94653856
Review-Url: https://codereview.chromium.org/2117773002
7 files changed