path_extrude_fn: extrude a variable shape over a path

[bendlas]

I wanted an extrusion function that could change the shape over the length of the path.

This design modifies the source for path_extrude to take a shape function and interpolates it between 0 and 1 over the path. The original path_extrude is recovered as a parameterization of that function (with a constant shape function).

The number of shape points has to be equal between all return values of the passed shape function.

Happy to make changes, like changing the interpolation from 0->1 to path point space, or adding documentation. But I'd first like to get feedback on whether you'd like to take this approach, and what to do with it ...

Here is an example of what can be achieved with that: