ccpr: Normalize the cubic inflection function instead of its roots

When solving for KLM, switches back to normalizing the cubic's
inflection function rather than both individual roots. Also performs
some general code clean up for SkClassifyCubic.

Bug: skia:
Change-Id: Id513e7e02c50a8709f3eccf92fad9e5134d73d83
Reviewed-on: https://skia-review.googlesource.com/121201
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
3 files changed