Add query to GrXPFactory about coverage-as-alpha optimization

This will be needed to have GrDrawOps that haven't yet built pipelines.

Change-Id: If5292aaa5dc9f98dccbe27be98960b630332158d
Reviewed-on: https://skia-review.googlesource.com/9480
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
7 files changed