gst/avi/gstavidemux.c: remove dead code, tweak debugs statements, add comments, use _uint64_scale instead _uint64_sca...

Original commit message from CVS:
* gst/avi/gstavidemux.c: (gst_avi_demux_index_entry_for_time),
(gst_avi_demux_src_convert), (gst_avi_demux_handle_src_query),
(gst_avi_demux_peek_chunk), (gst_avi_demux_parse_subindex),
(gst_avi_demux_read_subindexes_push),
(gst_avi_demux_read_subindexes_pull), (gst_avi_demux_parse_stream),
(gst_avi_demux_parse_index), (gst_avi_demux_stream_index),
(gst_avi_demux_sync), (gst_avi_demux_next_data_buffer),
(gst_avi_demux_massage_index),
(gst_avi_demux_calculate_durations_from_index),
(gst_avi_demux_stream_header_pull), (gst_avi_demux_do_seek),
(gst_avi_demux_handle_seek), (gst_avi_demux_process_next_entry),
(gst_avi_demux_stream_data), (gst_avi_demux_loop):
remove dead code, tweak debugs statements, add comments, use
_uint64_scale instead _uint64_scale_int when using guint64 values,
small optimizations, reflow some error handling