Detect coincident points when drawing arcs with rounding.

[jasondavies]

When rounding is used, it's possible for arc() to generate empty arcs in the case where the start and end points are almost coincident, and become coincident after rounding is applied.

This adds a check for coincident points after rounding is applied, and splits the arc into two if coincident points are detected.

Fixes #38.

[mbostock]

Thanks for the fix! And hi Jason! 👋 Two small nits.

[jasondavies]

It might also be worth generating a set of visual tests to double-check ccw={true, false} and values of a0, a1 where abs(a1 - a0) is some multiple of 2π, in case there's a faulty assumption with da /= 2.

[jasondavies]

I think my assumptions are correct. After these lines:

// Does the angle go the wrong way? Flip the direction.
if (da < 0) da = da % tau + tau;

// Is this a complete circle? Draw two arcs to complete the circle.
if (da > tauEpsilon) {
  this._append`A${r},${r},0,1,${cw},${x - dx},${y - dy}A${r},${r},0,1,${cw},${this._x1 = x0},${this._y1 = y0}`;
}

We can be sure that 0 ≤ da ≤ tauEpsilon, hence it's safe to use da /= 2 to determine the midpoint of the arc.