Method sampleAtD

src/main/java/field/graphics/FLine.java:1372–1376

returns a position on this line that's `distance` along it: specifically when `distance=0` then we're at the start of the line, when `alpha=0.5` then we are in the middle, when `distance=line.length()` we're at the end.

(double distance)

	/**
	 * how long is this line?
	 */
	public float length() {
		FLinesAndJavaShapes.Cursor c = cursor();
		return c.lengthD();
	}