Pull out sksl emission logic for per-edge aa from GrTextureOp

Bug: skia:
Change-Id: Iba068276e618d7d962b0b4ab059541f3b431539b
Reviewed-on: https://skia-review.googlesource.com/c/165528
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
3 files changed