Add support for arc-length metric in variable width stroke sample

I left in the ability to swap between using % arc length versus dividing
t evenly up across path segments so we can easily visually compare the
effect.

Change-Id: Id83792aa9e22fd5464e956092bac0baec389ffed
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330103
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
1 file changed