Implement a -filters option, listing all the available libavfilter

filters.
Currently filters are not registered, so the option will show none.

Originally committed as revision 20807 to svn://svn.ffmpeg.org/ffmpeg/trunk

cmdutils.h

@@ -163,6 +163,12 @@ void show_formats(void);
  */
 void show_codecs(void);
 
+/**
+ * Prints a listing containing all the filters supported by the
+ * program.
+ */
+void show_filters(void);
+
 /**
  * Prints a listing containing all the bit stream filters supported by the
  * program.