function buffer(size, num, dest) {
  if (dest === undefined) {
    dest = [0, 0];
  }
  dest[0] = size[0] + 2 * num;
  dest[1] = size[1] + 2 * num;
  return dest;
}
Returns a buffered size.
@param {Size} size Size.
@param {number} num The amount by which to buffer.
@param {Size} [dest] Optional reusable size array.
@return {Size} The buffered size.