Don't dither const shaders that don't have a GP override

https://gold.skia.org/detail?test=gradients_no_texture&digest=008c796df4da7fbb6718e96db2c36c90

If paintFP was only non-null because it came from the main SkShader, we
should defer to SkPaintPriv's check that the shader is not a constant.
We don't have a isConstant() for FPs so this assumes that if the GP
overrides it with a new FP it probably is doing something spatially
interesting and does need the dithering.

Bug: skia:12516
Change-Id: I7935ba8f0baf91e7b7ba07280452871c60ffd3e8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/509996
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed