Implement and document intersection

I haven't seen intersection code implemented in this
fashion. There is no round-off error in determining if
segments cross, but the resulting point is rounded.

I will build additional tests in upcoming CLs.

Bug: b/297220808
Change-Id: I764401c8a2badba7aeddd37ef2f58b70a666d592
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/746497
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed