Efficient draw

Rewrite of renderer code 10+ times speedup. Not sure why but it seems to slow down the solver by a factor of about 3 (this might be a measurement issue). The sum is down to a little over 0.005 for both render and solver (in the original code with NSMutableArray it was over 0.05 so the combination has now been sped up by a factor to ten.

The improvement is from directly writing the pixel array rather than drawing single pixels to a context  with drawRect.