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

TODO: Support more precision on 16-bit RGB pngs

BUG=skia:

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

Change-Id: I89dfef3b4887b9c4895c17309933883ab90ffa4d
Reviewed-on: https://skia-review.googlesource.com/6260
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>
13 files changed