Mp4Muxer caches the samples and then writes them in batches.
The new API allows disabling the batching and writing sample
immediately.
PiperOrigin-RevId: 689352771
This is done by reserving some space for moov box at the start of the file and writing it there if it fits. If it doesn't fit, then it is written at the end of the file.
PiperOrigin-RevId: 643944698