Reject horizontal edges in FDot6 precision

This simplifies the slope computation because we don't have to check dy
!= 0.

BUG=skia:

Change-Id: I5c09a9217ceed65f81f9d82cb045e33a70218077
Reviewed-on: https://skia-review.googlesource.com/7180
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
1 file changed