[Fluxus] stack tracing, pdata-exists? and extrusion fixes

[Dave Griffiths]

Hi all,

Some small commits:

A stack trace is now printed out with task errors including context,
source files and line numbers. This makes errors in every-frame much
easier to debug.

(pdata-exists?) returns true if a pdata array is present on a primitive
- eg:

(with-primitive (build-cube)
  (when (pdata-exists? "p") 
    (display "we have positions!") (newline)))
  (pdata-add "myarray" "v")
  (when (pdata-exists? "myarray") 
    (display "we have myarray!") (newline))))

Also some small fixes for animated extrusions to make them smoother.

cheers,

dave