アニメーションカーブで定義されるすべてのキー。
This lets you clear, add or remove any keys from the array.
If keys are not sorted by time, they will be automatically sorted on assignment.
Note that the array is "by value", i.e. getting keys returns a copy of all keys
and setting keys copies them into the curve.
関連項目: Keyframe 構造体、AddKey、RemoveKey 関数 .