Simplify calculation of the path gap.

Simplify the gap calculation code. Mainly avoid passing
the interval all over the place. Change code from handling
both horizontal and vertical to just horizantal.

Change-Id: Idb364da1e489e6a95d9b6d6dd97c4f6e85900c42
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/216691
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed