Make edge equations be flat interpolated in GrTextureOp

This only applies when the interpolants-are-inaccurate workaround is in play

Change-Id: I1cd8cbbfdc6f7f8f1764eeb588b2962d2d115340
Reviewed-on: https://skia-review.googlesource.com/104020
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed