[canvaskit] Add relative SkPath operators

Like with the non-relative forms, we want to be able to
chain these together, but not leak the SkPaths (which happens
if the C++ side returns this). Thus, we have to add in
the JS glue to return the "JS this".

Change-Id: Ic640b84f6c09c1d931ad44bc403b14bb0d0893a2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/251960
Reviewed-by: Hal Canary <halcanary@google.com>
5 files changed