Make GrWangsFormula::conic take "precision" instead of "tolerance"

Rearranges the algebra to use "precision". This makes it consistent
with the other formulas and removes the need for the caller to think
about inverting its precision value.

Bug: skia:10419
Change-Id: I186d03393952983e86b0bef69e1f89f86cdbb423
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/414616
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
7 files changed