MediaSource should not require Sync

[Herschel]

#98 added a Sync bound to the MediaSource trait and others, but I'm not sure this is the best idea. This prevents the use of a thread-unsafe MediaSource in a single threaded scenario where it is known there will be no data races.

[pdeljanov]

Hi @Herschel,

Sorry for the late response.

Yes, in retrospect I do feel like this was not the best change to make. I think I will probably be reverting this for 0.6.

Do you have a MediaSource that's running into this problem?

[Herschel]

No worries! Generally it's not a huge issue on the user side, and simply involves adding a Sync trait bound as appropriate. But because Sync is transitive, this can require digging quite deep to ensure the trait is propagated properly. In Ruffle's case, this required us to add some Sync bounds to both our core code and some other decoders (ruffle-rs/ruffle@24f0023). It seems like other users had to do so as well (RustAudio/rodio@21adce6).

So mostly an annoyance in practical cases because most things are Sync anyway, but it could be problematic if someone wanted to use the symphonia types in a non-Sync scenario, such as in a type with non-thread safe interior mutability like RefCell.