plumb cubic params into gpu backend

Tiny diffs seen, but not sure how to avoid those and keep the code
path simple -- so just plan/hope to rebaseline as needed.

Change-Id: Id8ff7e85a6e70785592f76118a32def2d61599ec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/314076
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
13 files changed