Mp4 crate/video types stores their content twice in RAM

[Wumpf]

Right now we effectively hold an mp4 blob at least twice: Raw in the (arrow) store and then again upon parsing in our mp4 object.
All larger pieces should just be pointers back to the raw blob in order to conserve memory.

Also general cleanup of mp4 crate is in order, this is a good opportunity.

This won't fly for streaming mp4 easily, but for full mp4 it should be doable to do indices into the blob.

[emilk]

Does arrow bottom out in https://crates.io/crates/bytes ? That would be useful for storing RC-counted references across crates.

Otherwise we have to consider a few different alternatives:

• re_mp4 uses bytes: Range<usize> instead of Vec<u8>
• re_mp4 has some sort of trait ByteSource with a Box<dyn ByteSource> that we then implement for Blob
• ???

[emilk]

I think the best path forward is to change re_mp4 to store byte ranges (Range<usize>) rather than the raw bytes, and then have re_video (or re_renderer) have some sort of trait ByteSource { fn read(&self, bytes: Range<usize>) -> Option<Vec<u8>> } that we then can wrap around a Blob.

It should be less than a days worth of work.