Add F16 and gamma correct support to webp encoder

BUG=skia:

Change-Id: Ib788466fa1e2bed26e7ffd8f03bee42930eae1d6
Reviewed-on: https://skia-review.googlesource.com/7425
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
4 files changed