Set the "mutable format" bit on sRGB images at creation time.

This allows us to later create an image view with a linear format, for
legacy-mode non-sRGB access.

No actual API change (just a comment change in public header).

TBR=bsalomon@google.com
BUG=skia:5308
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1981333004

Review-Url: https://codereview.chromium.org/1981333004
2 files changed