gst/mxf/: Rewrite parsing of descriptors to go top-down instead of bottom-up which...

gst/mxf/: Rewrite parsing of descriptors to go top-down instead of bottom-up which makes it possible to have the buff...

Original commit message from CVS:
* gst/mxf/mxfaes-bwf.c:
(mxf_metadata_wave_audio_essence_descriptor_handle_tag):
* gst/mxf/mxfaes-bwf.h:
* gst/mxf/mxfdemux.c:
(gst_mxf_demux_handle_metadata_generic_descriptor),
(gst_mxf_demux_handle_metadata_file_descriptor),
(gst_mxf_demux_handle_metadata_multiple_descriptor),
(gst_mxf_demux_handle_metadata_generic_picture_essence_descriptor),
(gst_mxf_demux_handle_metadata_cdci_picture_essence_descriptor),
(gst_mxf_demux_handle_metadata_mpeg_video_descriptor),
(gst_mxf_demux_handle_metadata_generic_sound_essence_descriptor),
(gst_mxf_demux_handle_metadata_wave_audio_essence_descriptor),
(gst_mxf_demux_handle_header_metadata_resolve_references),
(gst_mxf_demux_handle_metadata):
* gst/mxf/mxfmpeg.c:
(mxf_metadata_mpeg_video_descriptor_handle_tag):
* gst/mxf/mxfmpeg.h:
* gst/mxf/mxfparse.c: (mxf_partition_pack_parse),
(mxf_primer_pack_parse), (mxf_metadata_preface_parse),
(mxf_metadata_identification_parse),
(mxf_metadata_content_storage_parse),
(mxf_metadata_essence_container_data_parse),
(mxf_metadata_generic_package_parse), (mxf_metadata_track_parse),
(mxf_metadata_sequence_parse),
(mxf_metadata_structural_component_parse),
(mxf_metadata_descriptor_parse),
(mxf_metadata_generic_descriptor_handle_tag),
(mxf_metadata_generic_descriptor_reset),
(mxf_metadata_file_descriptor_handle_tag),
(mxf_metadata_generic_sound_essence_descriptor_handle_tag),
(mxf_metadata_generic_picture_essence_descriptor_handle_tag),
(mxf_metadata_cdci_picture_essence_descriptor_handle_tag),
(mxf_metadata_multiple_descriptor_handle_tag),
(mxf_metadata_locator_parse):
* gst/mxf/mxfparse.h:
Rewrite parsing of descriptors to go top-down instead of bottom-up
which makes it possible to have the buffer data non-writable.
Improve debugging a bit.