respect srgb gamma when building mips

Proposed policy:
- If the target is *legacy* (e.g. L32/PMColor) ignore gamma
- If the target is S32/F16 respect gamma

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2029373004

Review-Url: https://codereview.chromium.org/2029373004
19 files changed