I'd been assuming that the OS always copied our USB transfer buffer at submit-time. Turns out that this is true for Linux but not for Windows or Mac. This caused tearing when fcserver received a new frame while previous frame(s) were still queued in the USB host controller.

Fixed this by copying to a per-transfer buffer on Windows and Mac. Linux is unaffected. We avoid performing an additional copy there.

This lets us uniquely identify a particular device connection instance.

This was causing deadlocks all over on Windows. Now things seem to be
running smoothly!

Almost fast enough.. just 1.3ms more to shave off per frame.

Originally I wasn't planning on doing this, but since I need to reclaim some RAM anyway, might as well just start hacking out everything we don't need...