Add toy variable-width stroker sample

- Currently supports single-segment paths (lines or quads). Doesn't
  handle joins.
- Cubics are easy to add but not supported yet.
- No effort was made on performance optimization.
- Will likely be relocated eventually into an experimental SkPathEffect.

Change-Id: I35073d1d9dbc03a5423fda3bb20da005964c97d6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/329256
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
3 files changed