[fluxus] particle sprites fix

[Dave Griffiths]

just committed a fix for the sprites, they now always orient themselves in
camera space, and face forward so you can actually see them with backface
culling on :)

they also have "s" pdata for individually scaling the sprites

(define (init n)
    (pdata-set "p" n (vector (flxrnd)(flxrnd)(flxrnd)))
    (pdata-set "s" n (vmul (vector (flxrnd)(flxrnd)(flxrnd)) 0.2))
    (if (< n 1)
        0
        (init (- n 1))))

(clear)
(define p (build-particles 10))
(grab p)
(init (pdata-size))
(ungrab)
(show-axis 1)