Add an enum layer of indirection for stock raster pipeline stages.

This is handy now, and becomes necessary with fancier backends:
   - most code can't speak the type of AVX pipeline stages,
     so indirection's definitely needed there;
   - if the pipleine is entirely composed of stock stages,
     these enum values become an abstract recipe that can be JITted.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2782
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Change-Id: Iedd62e99ce39e94cf3e6ffc78c428f0ccc182342
Reviewed-on: https://skia-review.googlesource.com/2782
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
8 files changed