sketch SkVMBlitter

I'm having a hard time implementing SkRasterPipeline with SkVM,
so maybe it makes sense to try stepping over it instead?

Have not done much timing, except that it's noticeably slower
to generate GMs with this blitter turned on than turned off:
use of uniforms is forcing us into the interpreter.

I decided to start off with a perfect div255()... I'm curious to
see if we can live with perfect instead of an approximation.

Change-Id: I752f4a32ebe32ca550c9762aaad9ad86670b79d5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/231117
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
4 files changed