add relative segment methods

Bug: skia:9000
Change-Id: I1e7a4f3c799b7f6cd20e415880300a577a5a3c16
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/307346
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed