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/cmdutils.h

@@ -371,11 +371,13 @@ int check_stream_specifier(AVFormatContext *s, AVStream *st, const char *spec);
  * @param codec The particular codec for which the options should be filtered.
  *              If null, the default one is looked up according to the codec id.
  * @param dst a pointer to the created dictionary
+ * @param opts_used if non-NULL, every option stored in dst is also stored here,
+ *                  with specifiers preserved
  * @return a non-negative number on success, a negative error code on failure
  */
 int filter_codec_opts(const AVDictionary *opts, enum AVCodecID codec_id,
                       AVFormatContext *s, AVStream *st, const AVCodec *codec,
-                      AVDictionary **dst);
+                      AVDictionary **dst, AVDictionary **opts_used);
 
 /**
  * Setup AVCodecContext options for avformat_find_stream_info().