harden line clipper against slight numerical drift.

Bug: skia:7491
Change-Id: I8d2b7cbfb580c0aabc76a72dfb317c35db43b329
Reviewed-on: https://skia-review.googlesource.com/113425
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed