specialize drawAtlas for raster backend

2x faster than calling drawVertices as the impl.

Lots more to do in future CLs
- much of the time is spent in malloc, as we cons-up private shaders.
  we *could* create a private shader and wack its data for each draw
  (breaking the immutable contract, but that may be ok for a raster-only
  internal-use shader...)
- also spend time building the pipeline for each draw, even though
  all that has changed is a color payload (and the ctm). A custome
  stage(s) that exposed its private data could be reused with new
  data...

Bug: skia:
Change-Id: I0ff15155e37c0af7931abd34c0883701a47a048a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/203168
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
6 files changed