[jitter] big thanks

Here's the main loop from xray.jit.water.  prev is the previous matrix
in time.  Essentially there's a spatial weighting of the neighboring 8
cells and a temporal weighting with the previous value in the location
being calculated.  It should be straightforward to implement as a
shader which would be a cross of convolve and slide since it needs
temporal feedback and spatial weighting.

width  = prev_minfo->dim[0];
		height = prev_minfo->dim[1];
		
		incolspan = in1_minfo->dimstride[0];
		inrowspan = in1_minfo->dimstride[1];
		outcolspan = out_minfo->dimstride[0];
		outrowspan = out_minfo->dimstride[1];
		prevcolspan = prev_minfo->dimstride[0];
		prevrowspan = prev_minfo->dimstride[1];
		
		if (in1_minfo->type==_jit_sym_float32) {
			timestep = x->timestep;
			damping = x->damping;
			timeSpaceFactor = (timestep*timestep)/(x->spacestep*x->spacestep);
			dampFactor = 1 - damping*timestep;
		
			for(i=1; i < height-1; i++) {
				fip1_up = (float *)(bip1 + (i-1)*inrowspan);
				fip1 = (float *)(bip1 + i*inrowspan);
				fip1_down = (float *)(bip1 + (i+1)*inrowspan);
				fip2 = (float *)(bip2 + i*inrowspan);

				prev_fp = (float *)(prev_bp + i*prevrowspan);
				fop = (float *)(bop + i*outrowspan);
			
				for(j=1; j < width-1; j++) {
					fop[j] = fip1[j] + dampFactor*(fip1[j] - prev_fp[j] )
						- timeSpaceFactor*fip2[j]*(4*fip1[j] - fip1[j-1] - fip1[j+1] -
fip1_up[j] - fip1_down[j]
						+ 0.5*(4*fip1[j] - fip1_up[j-1] - fip1_up[j+1] - fip1_down[j-1]
- fip1_down[j+1] ));
					
					CLIP(fop[j], -1, 1);
				}
			}
		}