Avoid passing gradient-layout Y coordinate to colorizer.

The Y coordinate isn't meant to be interpreted as a coordinate. It's
used as a side channel for the discard signal.

Change-Id: Idbdd470c78c49a8b0a28eb05a93905c95b3282d1
Bug: skia:10548
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/307297
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
4 files changed