Ensure that dynamic info updates, even on static files, only update the
exact track they apply to, by atomically assigning the userInfo property
before opening the decoder, so that callbacks to the player indicate the
correct track and don't assume it's the one that's currently visibly
playing. Fixes start of track metadata notifications from overwriting
the previously playing track.
Signed-off-by: Christopher Snowhill <kode54@gmail.com>
120a93465e