从ALAsset中获取照片的原图并转换成NSData

一、有损

将其转换成UIImage对象，再通过UIImageJPEGRepresentation或UIImagePNGRepresentation方法转成NSData

这种方法不推荐，因为这个其实对图片进行了压缩转换，不再是图片的原始数据（像如果是gif动态图会变成静态图片），同时照片原来附带的拍摄信息什么的也会丢失。

let myAsset = ALAsset()
guard var representation =  myAsset.defaultRepresentation() else {
    return
}
var image = UIImage(cgImage:representation.fullResolutionImage().takeUnretainedValue())
var data = image.jpegData(compressionQuality: 1.0)

一、无损

直接复制图片的字节数据

let myAsset = ALAsset()
guard var representation =  myAsset.defaultRepresentation() else {
    return
}
var imageBuffer = UnsafeMutablePointer<UInt8>.allocate(capacity: Int(representation.size()))
var bufferSize = representation.getBytes(imageBuffer, fromOffset: 0, length: Int(representation.size()), error: nil)
var data:NSData =  NSData(bytesNoCopy:imageBuffer ,length:bufferSize, freeWhenDone:true)