move all memset() logic into blitter

It's cute in compile_pipeline(), but as before, clearer and simpler in the blitter.

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

Change-Id: Ib83ff097e4e057e72aed785797e6ac0029ca5dbf
Reviewed-on: https://skia-review.googlesource.com/5399
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed