function cross(a, b, dst) {
    dst = dst || new VecType(3);

    dst[0] = a[1] * b[2] - a[2] * b[1];
    dst[1] = a[2] * b[0] - a[0] * b[2];
    dst[2] = a[0] * b[1] - a[1] * b[0];

    return dst;
  }
Computes the cross product of two vectors; assumes both vectors have
three entries.
@param {module:twgl/v3.Vec3} a Operand vector.
@param {module:twgl/v3.Vec3} b Operand vector.
@param {module:twgl/v3.Vec3} [dst] vector to hold result. If not new one is created..
@return {module:twgl/v3.Vec3} The vector a cross b.
@memberOf module:twgl/v3