Handle looping in sfx thread.
Removed unnecessary stop of music (music will stop anyway once the
buffer is finished, but if music is completely threaded it might
just keep on playing).
Moved sfx thread update to happen after graphics code is finished.
This improves performance since threads in graphics code do not
compete with threads in sfx handling anymore.
Merge branch 'master' of github.com:supertuxkart/stk-code