Wrong resulting file duration when merging VBR files

[Aquassaut]

This comes from ffmpeg not computing the concat'd file size correctly. It will be difficult to handle after ffmpeg if we want to retain proper streaming. Three possible solutions:

• Hope that something will fix ffmpeg (hint: -write_xing 0 is not a fix).
• Maybe we could cheat and precalculate the duration with some other tool (sox for instance) and write our own VBR header. This seems like it could backfire easily and build a ton of unneeded complexity.
• Stop direct-streaming the output of ffmpeg if we detect a VBR input, and instead keep it in a temp location, fix the vbr header with something like vbrfix, and then send it. This seems like the easiest and most probable solution, but it would involve massive speed decrease.