Add tangents_to_point for QuadBez

[Keavon]

Follows up #288 which added tangents_to_point() to CubicBez, by adding it to QuadBez now as well.

[Keavon]

I'd like a double check on the math here, both in terms of its correctness (I'm not certain of it) and its formatting (is it more faithful to the canonical Bézier equations to reverse the orders of the terms, swap the order of the cross product and negate it, etc.?). This mostly arose from pattern recognition based fiddling and I wasn't able to quite grasp its relationship with the Bézier equations on Wikipedia. So please confirm this is right.

[waywardmonkeys]

I think this would be a little clearer:

      // Coefficients of (c + b t + a t^2) x (b + 2 a t) = 0.
      let c2 = b.cross(a);
      let c1 = 2.0 * c.cross(a);
      let c0 = c.cross(b);

[jneem]

I'd drop the "= 0" in the comment, but otherwise I agree that it's easier to follow this way.

I'd also maybe add a comment like this near the top:

// Reparameterize `self - p` as `a t^2 + b t + c`

[waywardmonkeys]

should grow some tests, these are from Codex and would be cleaner if another PR (#556) of mine lands (since they wouldn't have to call .deriv().eval(t).to_vec2() but could call .tangent(t):

  #[test]
  fn quadbez_tangents_to_point_recovers_known_t() {
      let q = QuadBez::new((0.0, 0.0), (1.0, 2.0), (3.0, 0.0));

      for &t in &[0.0, 0.2, 0.5, 0.8, 1.0] {
          let pt = q.eval(t);
          let tan = q.deriv().eval(t).to_vec2();

          // Pick points along the tangent line at t.
          for &scale in &[-2.0, -0.5, 0.5, 3.0] {
              let p = pt + scale * tan;
              let roots = q.tangents_to_point(p);
              assert!(
                  roots.iter().any(|r| (*r - t).abs() <= 1e-12),
                  "expected t={t} in roots {roots:?} for point {p:?}"
              );
          }
      }
  }

  #[test]
  fn quadbez_tangents_to_point_finds_two_symmetric_solutions() {
      let q = QuadBez::new((-1.0, 0.0), (0.0, 1.0), (1.0, 0.0));

      // This point is the intersection of the tangent lines at t = 0.25 and t = 0.75.
      let roots = q.tangents_to_point((0.0, 0.625).into());

      assert_eq!(roots.len(), 2, "expected two roots, got {roots:?}");
      assert!((roots[0] - 0.25).abs() <= 1e-12, "roots = {roots:?}");
      assert!((roots[1] - 0.75).abs() <= 1e-12, "roots = {roots:?}");
  }

  #[test]
  fn quadbez_tangents_to_point_handles_endpoint_tangents() {
      let q = QuadBez::new((0.0, 0.0), (1.0, 2.0), (3.0, 0.0));

      let p0 = q.p0 + 2.0 * q.deriv().eval(0.0).to_vec2();
      let roots0 = q.tangents_to_point(p0);
      assert!(
          roots0.iter().any(|r| r.abs() <= 1e-12),
          "expected root near 0, got {roots0:?}"
      );

      let p1 = q.p2 - 1.5 * q.deriv().eval(1.0).to_vec2();
      let roots1 = q.tangents_to_point(p1);
      assert!(
          roots1.iter().any(|r| (*r - 1.0).abs() <= 1e-12),
          "expected root near 1, got {roots1:?}"
      );
  }

  #[test]
  fn quadbez_tangents_to_point_can_return_no_solutions() {
      let q = QuadBez::new((-1.0, 0.0), (0.0, 1.0), (1.0, 0.0));

      // For this symmetric arch, points on the y-axis above y = 1 have no real
  tangent lines.
      let roots = q.tangents_to_point((0.0, 1.5).into());
      assert!(roots.is_empty(), "expected no roots, got {roots:?}");
  }

  #[test]
  fn quadbez_tangents_to_point_near_linear_curve() {
      let q = QuadBez::new((0.0, 0.0), (0.5, 1e-9), (1.0, 0.0));
      let t = 0.3;
      let p = q.eval(t) + 0.75 * q.deriv().eval(t).to_vec2();
      let roots = q.tangents_to_point(p);

      assert!(
          roots.iter().any(|r| (*r - t).abs() <= 1e-9),
          "expected t={t} in roots {roots:?}"
      );
  }

[waywardmonkeys]

Needs tests and a rebase forward.

[waywardmonkeys]

I think this would be a little clearer:

      // Coefficients of (c + b t + a t^2) x (b + 2 a t) = 0.
      let c2 = b.cross(a);
      let c1 = 2.0 * c.cross(a);
      let c0 = c.cross(b);