[graphite] Add conical gradient shader

This is largely a re-implementation of the logic in
src/gpu/ganesh/gradients/GrGradientShader.cpp and
src/shaders/gradients/SkTwoPointConicalGradient.cpp. For simplicity,
most everything is in the shader now, but with an eye towards moving
computations that don't use sk_FragCoord into SkKeyHelpers.cpp.

Bug: skia:13302
Change-Id: I73d2e1f6c6ac0580aa897ffff3cfa87f4ed62b11
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/539569
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
4 files changed