added support for sk_ClipDistance

Change-Id: I14fd921cace23ab37a1e5e9d5da94d92d3688680
Reviewed-on: https://skia-review.googlesource.com/8225
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
5 files changed