relax quadratic binary search test

Extreme implicit quartic equations solve to roots that are different
enough that they appear to have failed. In this case, fall back on
binary searching to find an intersection.

Relax the condition when this happens; don't give up just because the
computed implicit root points aren't remotely the same.

TBR=reed
BUG=skia:2808

Author: caryclark@google.com

Review URL: https://codereview.chromium.org/456383003
4 files changed