Initialize test textures for ProcessorOptimizationValidationTest.

Ensure the data in the RGBA texture is premul.

Reenable the "modulation" optimization flag on GrAlphaThresholdFragmentProcessor.

Change-Id: I49ef215cee83ea74526c589bcee4bc215bc28ba4
Reviewed-on: https://skia-review.googlesource.com/8323
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed