Add tolerance to quadratic and linear cubic detection

Otherwise these can be misclassified as cusps since there is already
tolerance in the discriminant test.

BUG=skia:

Change-Id: Id02c12f671714cebf799953ebed5335ee4c4d52a
Reviewed-on: https://skia-review.googlesource.com/9355
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed