ffmpeg: store value of -filter and -filter_script per-stream option in OutputStream struct

Avoid the need for multiple potentially inconsistent access operations,
slightly factorize code.

ffmpeg.h

@@ -357,6 +357,8 @@ typedef struct OutputStream {
 
     OutputFilter *filter;
     char *avfilter;
+    char *filters;         ///< filtergraph associated to the -filter option
+    char *filters_script;  ///< filtergraph script associated to the -filter_script option
 
     int64_t sws_flags;
     AVDictionary *opts;