revamped gfx PrimList

PrimList now contains four specific rendering primitives: points, lines,
spheres and cylinders; lines and cylinders accept two different colors
for each endpoint, and cylinders support two different radii for their ends.
The rendering mode no longer has any effect, also, it is no longer possible
to change the radius of spheres and cylinders once they are added.

The python interface is much more sophisticated, using named arguments
for optional parameters.

The low-level cylinder routine in IndexedVertexArray was adjusted
accordingly