VmhdBox flags issue

[mattc1170]

I have run into an issue trying to play H.264-encoded .mp4 files created with this crate on Android. After researching the issue and comparing files generated by both mp4-rust and ffmpeg, I found that the vmhd box flags parameter needs to be set to 1 for proper playback on Android.

I have implemented a fix by implementing Default::default() for VmhdBox with flags set to 1 and will post a PR.