use raster-pipeline in readPixels

1. tons of 565 diffs in gm, so skipping that for now
2. 32bit premul/unpremul conversions are slower than existing code, so for now use the pipeline after the existing special-cases.
BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5152
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot;skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD


Change-Id: I6ca43b6dd24434814f8f10cdaaabbaf396914d1a
Reviewed-on: https://skia-review.googlesource.com/5152
Commit-Queue: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
2 files changed