gst/avi/gstavidemux.*: Proper aggregation of each stream's GstFlowReturn in order to figure out whether the task shou...

Original commit message from CVS:
* gst/avi/gstavidemux.c: (gst_avi_demux_reset),
(gst_avi_demux_read_subindexes), (gst_avi_demux_parse_stream),
(gst_avi_demux_handle_seek), (gst_avi_demux_aggregated_flow),
(gst_avi_demux_process_next_entry), (push_tag_lists),
(gst_avi_demux_stream_data), (gst_avi_demux_loop):
* gst/avi/gstavidemux.h:
Proper aggregation of each stream's GstFlowReturn in order to figure out
whether the task should stop or not.
Don't send inline events before pushing out a NEW_SEGMENT, more
specifically for GST_TAG_EVENT.
Change a GST_ERROR to a GST_WARNING for a non-fatal situation in reading
sub-indexes.