ccpr: Clip quadratic hulls more efficiently

To minimize area, we should clip by the tangent line at maximum
height. Not mid-tangent. This is also easier to find.

Bug: skia:
Change-Id: I5099b62537f72e51e29bbced7998c0e588e287d9
Reviewed-on: https://skia-review.googlesource.com/119740
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
6 files changed