Support SkImageShader in SkRasterPipeline blitter

First of many CLs, I'm sure.

This handles 8888 or sRGB sources with an affine matrix, clamp/clamp tiling, and nearest-neighbor sampling only.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4906
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Change-Id: I99f7508852b3d44b6f52f7a0bee29a793af35c48
Reviewed-on: https://skia-review.googlesource.com/4906
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
6 files changed