Mod that handles negatives

Found the issue with negative region coordinates in blockplot:

```js
function mod (num, n) { return ( num < 0 ? (num % n) + n : num % n) }
console.log(mod(-32, 32)); // === 32

function mod (num, n) { return ((num % n) + n) % n }
console.log(mod2(-32, 32)); // === 0
```

So `r.0.-1.mca` would start counting at `32` and be instantly out of range.