Two zero-distance points annihilate each other

```
pbourke discovered that if you have pt 1 & 2 down and bring them close 
together, they merge into one point, and then MTVisualizer thinks there are 0 
points down (you can keep moving pts 1 & 2 and the screen looks like no points 
are down).

```

Original issue reported on code.google.com by `luke.hutch` on 22 Apr 2012 at 4:47
