ドラッグしているマウスのデルタ（位置の変化量）のフロート値を取得します（読出専用）。
This will autodetect the users preference for X / Y axis dragging and give you back
a float that is measured accordingly. This also handles modifier keys correctly, i.e.
the delta is already multiplied by acceleration.
See Also: acceleration, niceMouseDeltaZoom.