This re-works the subpath model and does transforming
of ports in JS rather than in C++; the latter was not easy
to conform to the Canvas Spec, especially for changing
transforms in the middle of a path.
Additionally adds jpeg to the available images it can produce.
Bug: skia:
Change-Id: I5a52ec341d4060198c8680aa4d3b85a26f77b6b9
Reviewed-on: https://skia-review.googlesource.com/c/172500
Reviewed-by: Kevin Lubick <kjlubick@google.com>
b9db3906d5