finish up 2pt conicals

In the end it turned out best to let the subclasses
modify the mask, rather than return how to do it.
This gave more flexibility about how to calcualte it.

Add negate(x), norm(x,y).

Change-Id: Ie17050037f0441becf06897fbe31587d6709009d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/267456
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
11 files changed