Remove XP override color.

The only use case for this was using the blend constant for LCD text. Now instead of overriding the op's color with an alpha we upload the alpha as a uniform.

This also removes two unused parameters from GrXferProcessor::getOptimizations.

Change-Id: I8268da9904a5d26649c6ae81a5705b0930893904
Reviewed-on: https://skia-review.googlesource.com/9221
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
9 files changed