extend clipper for lines, for uniformity, and so curves can call itself with lines

BUG=skia:

Change-Id: Id8c1fba6fcd2496802d3d17afe3f5c91bf5dfc33
Reviewed-on: https://skia-review.googlesource.com/7621
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed