Moving music streaming to a thread for desktop

[mgsx-dev]

This PR is an attempt to move music streaming to a thread for desktop backends. It still a work in progress, it almost work as expected but there are some corner cases to fix, maybe i'll need to change the synchronization strategy.

I only changed Lwjgl3 backend for now, i'll copy/paste to lwjgl2 backend once it's all clear.

I need to find time to finish it but i thought it was preferable to PR this WIP for discussion as a replacement of #6452.