audioconvert: Only considers first structure when fixating caps

Submitted by Sebastian Dröge @slomo

Link to original bug (#706884)

Description

This pipeline for example should negotiate to S32BE instead of S16LE:

gst-launch-1.0 audiotestsrc ! audioconvert ! capsfilter caps="audio/x-raw,format=S32LE" ! audioconvert ! capsfilter caps="audio/x-raw,format={S8, S16LE};audio/x-raw,format=S32BE" ! fakesink -v

But because the fixate function inside gstaudioconvert.c only looks at the first structure this does not happen. It should probably iterate over all structures after fixating each structure for itself, and then select the first best structure according to the same algorithm as it currently fixates every structure.