Add is_material function

It was inadvertently left out of all-in-one.lua when modularizing
snogray Lua support.
Handle AXIS+RADIUS style arguments in surface.ellipse

These are often more convenient than specifying two orthogonal radii
in the case of a simple disk.

Also add a "surface.disk" alias for surface.ellipse.
snogray.lua: Correctly handle rendering with alpha-channel
snogray.lua: Print info about output file
Add surface.box alias for surface.parallelepiped

Easier to spell... :]