Migrate the code to use the new parser API based on GstMpegVideoPacket.
Also try to optimize gst_mpegv_parse_process_config() by using more of
GstMpegVideoPacket and determining the extension_start_code_identifier
prior to calling the parser function for that extension packet.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>