[jitter] compositing matrices per pixel

I used to do it this way, but then jkc showed me the light: glue  
matrices in one wide matrix and use jit.repos.
I have an old example of this, but won't be able to post it until  
later tonight.


> #P newex 604 186 107 196617 jit.op @op == @val 3.;
> #P newex 549 223 65 196617 jit.op @op *;