Add a low level quadratic root solver

This is a solver based on W. Kahan's solver. It will be
used as part of Skia's multiple existing solvers.

Change-Id: Ib95c5516f1260e5394a8cc072987c14559bc84b6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/705225
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
3 files changed