Make GrSimpleMeshDrawOpHelper record whether coverage can be implemented as alpha

This also makes the blend table entry for opaque src over indicate a blend of (1, ISA) rather than (1, 0) to match the actual implementation of the global src-over XP.
Change-Id: I1b1f64d2546e4f0cf03c0239ce674d1baad655f6
Reviewed-on: https://skia-review.googlesource.com/18521
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
5 files changed