feature request: provide incremental API for mp4 decode/encrypt/encode

[lukseven]

We are currently using the following skeleton code for encrypting fragmented mp4 content per segment:

	mp4Data, err := mp4.DecodeFile(segReader)
	...
	for _, s := range mp4Data.Segments { // always 1 segment...
		for _, f := range s.Fragments {
			err = mp4.EncryptFragment(f, args.keyBytes, args.iv, args.ipd)
			...
		}
	}
	buf := bytes.Buffer{}
	err = mp4Data.Encode(&buf)
	...

We would like to use encryption on live content, where media segments and fragments are produced continuously. Latency is critical, so ideally we would want to encrypt and output fragments as soon as they become available in the source reader, without waiting for complete segments (with all of their fragments) to be available. Maybe something along these lines:

	var segReader io.Reader = ...
	var segWriter io.Writer = ...
	mp4.EncryptSegment(segReader, segWriter, args.keyBytes, args.iv, args.ipd)

[tobbee]

Hmm. mp4ff does not currently have a stream-processing mode. It does have lazy loading of mdat using io.ReadSeeker so if one provides an mp4 fragment with a moof + mdat it will only read the moof box at first, and then read the relevant byte ranges by seeking in mdat as needed.

For a pure streaming interface, I think an extra layer of buffering is needed to first gather and parse the relevant top-level moof box and then change the Seek operations to work on a buffer that is read progressively. I'll ponder this a bit.

[tobbee]

@lukseven Please have a look at #464 and tell if if what you need? The example is a bit barebone and uses an asset with an init segment at the start. If there is none, one needs to provide it at the side as in mp4ff-encrypt.