SkRasterPipelineBlitter: support A8
This adds support for loading and storing A8, then uses it in SkRasterPipelineBlitter.
I think this handles all dst formats now: A8, 565, 8888 (by policy, sRGB only) and F16.
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD
Change-Id: Id207f6e6c56b6bfcc301d77dd23e0959bb7afba8
Reviewed-on: https://skia-review.googlesource.com/6554
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>