function cross(a, b, dst) {
  dst = dst || new VecType(3);
  var t1 = a[2] * b[0] - a[0] * b[2];
  var t2 = a[0] * b[1] - a[1] * b[0];
  dst[0] = a[1] * b[2] - a[2] * b[1];
  dst[1] = t1;
  dst[2] = t2;
  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 of a cross b.
@memberOf module:twgl/v3