BUG: When in perspective mode a point behind the camera is transformed, the inverse transform puts it back into the wrong place.  This was causing some distance calculations to be bigger than they should have been and consequently the sqrt array lookup (set up to handle distances only as big as cells) was causing seg faults for large distances.  Fix that by ignoring any cell with a point behind the camera.

PERF: Skip the projection calculations for any cells outside of the clipping range.