[SkPathBuilder] Add rMoveTo

This is adapted from SkPath::rMoveTo.

Bug: b/40040287
Change-Id: Ib559199e1ee4c2f29d697a17e1a52fa33a96d9e2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1028776
Reviewed-by: Florin Malita <fmalita@google.com>
Auto-Submit: Eric Boren <borenet@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>