Some lowp refactoring

  1) Move a couple stages around in the enum to places
     that make more sense, and guass_a_to_rbga in the code too.

  2) mirror the SkRasterPipeline stage enum with either:
     LOWP(st): the stage is implemented in low precision
     TODO(st): the stage should be lowp, but isn't
     NOPE(st): the stage shouldn't be done in lowp.

  3) statically enforce that all stages are covered by one of
     LOWP, TODO, or NOPE.

Change-Id: I06c7a7e470663ef73bf652c1b65c0d3c89f0d767
Reviewed-on: https://skia-review.googlesource.com/63800
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
5 files changed