function copy(v, dst) {
    dst = dst || new VecType(3);

    dst[0] = v[0];
    dst[1] = v[1];
    dst[2] = v[2];

    return dst;
  }
Copies a vector.
@param {module:twgl/v3.Vec3} v The vector.
@param {module:twgl/v3.Vec3} [dst] vector to hold result. If not new one is created..
@return {module:twgl/v3.Vec3} A copy of v.
@memberOf module:twgl/v3