Add support for automatic texture mapping

[ianmackenzie]

To start:

• Planar (based on a sketch plane)
• Spherical (based on a frame)
• Cylindrical (based on a frame)

Would also need a way to specify the texture scale (meters per pixel? texture width? width and height?).

Should likely be implemented as functions that transform a Material - convert one that requires UV coordinates into one that doesn't.

[ianmackenzie]

Perhaps simpler: have some nice helpers for generating UV coordinates when building a mesh, e.g.

Mesh.planarUv : 
    Rectangle3d Meters coordinates
    -> Point3d Meters coordinates
    -> ( Float, Float )

Mesh.sphericalUv : 
    Frame3d Meters coordinates defines
    -> Point3d Meters coordinates
    -> ( Float, Float )