Add adjustable arrow barb location

We are dancing right on the edge of "noticeably slower." We have to be careful about the barb is actually drawn. This way seemed to perform the best and look the least ridiculous.
Would this be better as a plain object? Probably...

```javascript
barbPosition = {tail: 0, center: 0.5, head: 1.0}[barbLocation];
// ... handle undefined
```