Update the Gradient documentation.

Since this doc was written, we've made several changes to how gradients
work under the hood:

- Colorizers receive the t interpolant via explicit-coordinate sampling,
  not via sk_InColor
- Unused components passed to the colorizer are guaranteed to be zero
  instead of potentially arbitrary values
- We no longer have a dedicated colorizer that is responsible for
  sampling a generated texture map; GrTextureEffect is used directly
- Rewrote the section about how layout processors encode their return
  value to hopefully be more clear to a reader unfamiliar with gradients

Change-Id: I037b5718c9f67a1bece96a9b6a4daea79ccdcd80
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/311076
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
1 file changed