fftools/ffmpeg: rewrite checking whether codec AVOptions have been used

Share the code between encoding and decoding. Instead of checking every
stream's options dictionary (which is also used for other purposes),
track all used options in a dedicated dictionary.

fftools/ffmpeg_mux.c

@@ -864,6 +864,7 @@ void of_free(OutputFile **pof)
     av_freep(&mux->sch_stream_idx);
 
     av_dict_free(&mux->opts);
+    av_dict_free(&mux->enc_opts_used);
 
     av_packet_free(&mux->sq_pkt);