ffmpeg: Do not fail if a demuxer and decoder use the same option identifer

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -532,6 +532,15 @@ static void ffmpeg_cleanup(int ret)
     term_exit();
 }
 
+void remove_avoptions(AVDictionary **a, AVDictionary *b)
+{
+    AVDictionaryEntry *t = NULL;
+
+    while ((t = av_dict_get(b, "", t, AV_DICT_IGNORE_SUFFIX))) {
+        av_dict_set(a, t->key, NULL, AV_DICT_MATCH_CASE);
+    }
+}
+
 void assert_avoptions(AVDictionary *m)
 {
     AVDictionaryEntry *t;