function getVelocity(deltaTime, x, y) {
  return {
    x: x / deltaTime || 0,
    y: y / deltaTime || 0
  };
}
@private
calculate the velocity between two points. unit is in px per ms.
@param {Number} deltaTime
@param {Number} x
@param {Number} y
@return {Object} velocity `x` and `y`