Add support for Navigation Meshes

[davesmith00000]

Somewhat related to #804:

I think we need a way to define a 'nav mesh', which is a shape defining the space that can be travelled across (by a character or NPC).

How complicated this is, is open to interpretation and I think we could start with simple shapes, but I think the requirements are:

1. Can somehow define the space (Arbitrary* | Grid | Hex?)
2. Can define obstacles
3. Can combine and convert those two things into a mesh of possible points (maybe reuse the Delaunay triangulation code?)
4. Can use something like A* to find a good route through the points
5. Can convert the points to WayPoints for use with Waypoints: Add functions for piecewise linear interpolation #804

(* Polygons you can move in freely. Do we need a way to connect polygons / nav meshes? Grid's and Hex grid would do this naturally I think.)