void UpdatePivotOffset()
{
float px = _pivot.x * _contentRect.width;
float py = _pivot.y * _contentRect.height;
//注意这里不用处理skew,因为在顶点变换里有对pivot的处理
Matrix4x4 matrix = Matrix4x4.TRS(Vector3.zero, cachedTransform.localRotation, cachedTransform.localScale);
_pivotOffset = matrix.MultiplyPoint(new Vector3(px, -py, 0));
}