update flight path intercept code

[Pomax]

Stagger points along the flight path, past a transition, and (lerp-)target those (moving to next point(s) as we pass them), so that we get (back) on the flight path earlier.

[Pomax]

E.g.

[Pomax]

This should hopefully avoid situations like this:

[Pomax]

See the socketless extras dir, and the Processing flightpath sketch.

[Pomax]

More specifically, see the new getLineCircleIntersection function in `utils.js:

function getLineCircleIntersection(
  { x, y },
  { x: x1, y: y1 },
  { x: x2, y: y2 },
  r
) {
  // work relative to (0,0)
  x1 -= x;
  y1 -= y;
  x2 -= x;
  y2 -= y;
  const dy = y2 - y1;
  const dx = x2 - x1;
  const m = (dx * dx + dy * dy) ** 0.5;
  const A = x1 * y2 - x2 * y1;
  const d = (r * r * m * m - A * A) ** 0.5;
  return {
    x: x + (d * dx + A * dy) / (m * m),
    y: y + (d * dy - A * dx) / (m * m),
  };
}

[Pomax]

this will be covered by the new tutorial, and has been implemented in the improved branch