avidemux: short-circuit gst_avi_demux_src_convert() when parsing the index

avidemux: short-circuit gst_avi_demux_src_convert() when parsing the index

Don't call gst_avi_demux_src_convert() for each single index entry. Not
only do we already have the pointer to the stream context, we also know
the formats we want to convert from and to already, so we may just as
well use optimised conversion routines that bypass some of the checks
and lookups made in gst_avi_demux_src_convert().