scad_render_to_file can be used in REPL

It tries to use calling_module() which returns None, if we call scad_render_to_file(...) from the REPL. I change it, so it doesn't include the Python source code, if it cannot access it.

NOTE: There are other functions that use calling_module() and I think they will fail when called from the REPL. I think this only affects `use`, but there may be others.