Allow negative rates

Some inputs (e.g. RTSP) can provide negative rates (rewind) just fine

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
mjpeg: work around broken MIME separators in crappy firmwares

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
libvlc: libvlc_MediaSubItemTreeAdded event type added

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
MacOSX: fix engrish error