Improves Image Shaper demo.

Notably:

* FourPassImageShaper now sort of works - it is just doing two passes
(left and right),
but it works well enough for my needs.
* Fixes issue where images would be expanded if they were smaller than
the max icon size.
* Fixes basic arrow placement, at least when using the default edge
shape.
* Fixes bugs in how I was interpreting canvas paths in FXU; the
documentation is light.