restrict download / prefecting size to 1mb

[8secz-johndpope]

https://github.com/wxxsw/GSPlayer)](https://github.com/wxxsw/GSPlayer

https://github.com/wxxsw/GSPlayer/blob/257f1fca3860fdbbad112b0524a90985589437fd/GSPlayer/Classes/Download/VideoDownloader.swift#L2

https://github.com/wxxsw/GSPlayer/blob/257f1fca3860fdbbad112b0524a90985589437fd/GSPlayer/Classes/Loader/VideoPreloadManager.swift#L15
public var preloadByteCount: Int = 1024 * 1024 // = 1M