[Fluxus] nuke button + png saving commits

[Dave Griffiths]

Hi all,

Some more commits:

F6 now resets the entire interpreter and graphics system. This is useful
for clearing everything out when things are broken - not particularly
useful for the livecoding style of incremental changes, as it takes half a
second or so - but should be useful (and a bit clearer for beginners to
use).

(load-primitive) and (save-primitive) now work properly for png images.
(load-primitive "mypic.png") creates pixel primitives, calling
(save-primitive) while a pixel primitive is current will save a png file.
(I wanted to use fluxus to do some image offline processing)

The vector maths works for length 4 or length 3 vectors, so you can use
them to modify rgba colour values.

cheers,

dave