MMALCodec: Add an extra buffer to picture pool

I believe we need an extra buffer when seeking
as a picture remains on screen from old seek point.

See: http://trac.kodi.tv/ticket/17285

xbmc/cores/VideoPlayer/DVDCodecs/Video/MMALCodec.cpp

@@ -524,7 +524,8 @@ bool CMMALVideo::Open(CDVDStreamInfo &hints, CDVDCodecOptions &options)
   if (status != MMAL_SUCCESS)
     CLog::Log(LOGERROR, "%s::%s Failed to disable error concealment on %s (status=%x %s)", CLASSNAME, __func__, m_dec_input->name, status, mmal_status_to_string(status));
 
-  status = mmal_port_parameter_set_uint32(m_dec_input, MMAL_PARAMETER_EXTRA_BUFFERS, GetAllowedReferences());
+  // we need an extra buffer when seeking as a picture remains on screen from old seek point
+  status = mmal_port_parameter_set_uint32(m_dec_input, MMAL_PARAMETER_EXTRA_BUFFERS, GetAllowedReferences() + 1);
   if (status != MMAL_SUCCESS)
     CLog::Log(LOGERROR, "%s::%s Failed to enable extra buffers on %s (status=%x %s)", CLASSNAME, __func__, m_dec_input->name, status, mmal_status_to_string(status));