fix f16 encoding

We'll encode F16 .pngs at 16-bit linear, so don't to_srgb them.
We'll encode F16 .jpgs and .webps as 8-bit sRGB, so tag them as sRGB.

Change-Id: Ibc5b5e2d3aea68cdf1531026cbe8b6191f7e39ea
Reviewed-on: https://skia-review.googlesource.com/19498
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed