allow for skewed extrusion

[Nico Schlömer]

Currently, extrusion only supports the direction (0,0,1), and polygons are assumed to sit in the z=0-plane. This makes it impossible to create skewed extrusion for, e.g., oblique cones.

One possible solution for the interface would be to allow specification of the extrusion direction (perhaps defaulting to (0,0,1)), taking into account the length of the vector too.