function getVelocity(deltaTime, x, y) {
	    return {
	        x: x / deltaTime || 0,
	        y: y / deltaTime || 0
	    };
	}
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`