Add F16 support to SkPNGImageEncoder

BUG=skia:

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: Ifd221365a7b9f9a4a4fc5382621e0da7189e1148
Reviewed-on: https://skia-review.googlesource.com/6526
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
6 files changed