gst/avi/gstavidemux.c: Use scaling code for added precission and more correct stop position in case scale==0.

Original commit message from CVS:
* gst/avi/gstavidemux.c: (gst_avi_demux_src_convert),
(gst_avi_demux_handle_src_query), (gst_avi_demux_handle_src_event),
(gst_avi_demux_parse_file_header), (gst_avi_demux_stream_init),
(gst_avi_demux_parse_avih), (gst_avi_demux_parse_superindex),
(gst_avi_demux_parse_subindex), (gst_avi_demux_parse_stream),
(gst_avi_demux_stream_header), (gst_avi_demux_change_state):
Use scaling code for added precission and more correct stop
position in case scale==0.