Decouple Graphite and Shaders

The gradient shaders will need a follow-up to remove the mutable
SkBitmap that is used when there are more than 8 stops. While
migrating that code, I made a mistake and added an assert that helped
detect and fix it.

Bug: b/40045065
Change-Id: I44abcc089bcf7de3e381d28abd697d4f8c626b78
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/729258
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
39 files changed