Fix gpu lcd blending to semi-correctly handle alpha coverage

Bug: skia:6606
Change-Id: I16ccd97f5d047eb7fddfed5310bf669e7435ccdd
Reviewed-on: https://skia-review.googlesource.com/17370
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
6 files changed