skvm sweep gradients

Basically the same as SkRasterPipeline version.
Need to thread uniforms through for scale/bias.

Change-Id: I27fd91469d9108d155a751dad71047eded4b6692
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/267223
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
8 files changed