Function apply

apply(
t: Triangle,
fn: (p: Geometry.Point, label?: string) => Geometry.Point,
): Readonly<Triangle>
Applies fn to each of a triangle's corner points, returning the result.
Parameters
- t: Triangle
- fn: (p: Geometry.Point, label?: string) => Geometry.Point
Returns Readonly<Triangle>
Example: Add some random to the x of each corner
```
const t = apply(tri, p => {
 const r = 10;
 return {
   x: p.x + (Math.random()*r*2) - r,
   y: p.y
 }
});
```