fftools/ffmpeg: add support for multiview video

This extends the syntax for specifying input streams in -map and complex
filtergraph labels, to allow selecting a view by view ID, index, or
position. The corresponding decoder is then set up to decode the
appropriate view and send frames for that view to the correct
filtergraph input(s).

fftools/cmdutils.h

@@ -543,4 +543,6 @@ void remove_avoptions(AVDictionary **a, AVDictionary *b);
 /* Check if any keys exist in dictionary m */
 int check_avoptions(AVDictionary *m);
 
+int cmdutils_isalnum(char c);
+
 #endif /* FFTOOLS_CMDUTILS_H */