Use RasterPipeline to support full precision on 16-bit RGBA pngs

Reland of Original Change:
https://skia-review.googlesource.com/6260

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

Change-Id: I809984dd9af225103bfbe83492a17c19da7c5e40
Reviewed-on: https://skia-review.googlesource.com/6980
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
13 files changed