gst/qtmux/: Cut detour in sample description extension construction.

gst/qtmux/: Cut detour in sample description extension construction.

Original commit message from CVS:
* gst/qtmux/atoms.c: (build_esds_extension),
(build_mov_aac_extension), (build_jp2h_extension),
(build_codec_data_extension):
* gst/qtmux/atoms.h:
* gst/qtmux/fourcc.h:
* gst/qtmux/gstqtmux.c: (gst_qt_mux_audio_sink_set_caps),
(gst_qt_mux_video_sink_set_caps):
* gst/qtmux/gstqtmuxmap.c: (gst_qt_mux_map_format_to_header):
Cut detour in sample description extension construction.
Also actually implement ISO JPEG2000 mj2 format.