A rectangle's progression around a path:
Canvas paths -- i.e. the Path2D object -- don't have the flexibility of SVG paths for common animation tasks like finding the coordinates of a point at a distance along the path.
This Path object should provide a handy wrapper for the Path2D object which stores information about the path so that it can provide these features.
It will have all the methods like arc, lineTo, etc. of a regular Path2D object, and the
underlying Path2D object will be available by accessing the _path2d attribute.
However it will also provide the getInfoAtDistance method, which gives the X and Y values
of a point along the line, as well as an angle corresponding to the gradient of the tangent
to the line at that point. This is invaluable for animating shapes moving "around" a path,
e.g. trains moving along a track.
A PathFollower object is also provided, which creates a single-use iterable object to find
information about the points at incrementally greater distances. This provides just two
methods: advance(distance) and getInfo(). This is useful for animating a shape's motion
along a particular path.
Tests are provided in tests.html; please see the source comments for advice on where to
put breakpoints in order to see each test succeed.