rough sketch of shaders in SkVMBlitter

Change-Id: I5f96475a6dec26dde0bdf883153a1fd7c7e3b148
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/251409
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
4 files changed