从视频的左侧（在UIImagePickerController中的修剪工具栏开始，修剪功能不起作用。如果我们给定videoMaximumDuration并且视频长度超过videoMaximumDuration，则可以正常工作。
let imagePicker = UIImagePickerController()
imagePicker.delegate = self
imagePicker.sourceType = .photoLibrary
imagePicker.allowsEditing = true
//imagePicker.videoMaximumDuration = 60
imagePicker.mediaTypes = ["public.movie"]