lavfi: remove 'opaque' parameter from AVFilter.init()

It is not used in any filters currently and is inherently evil. If
passing binary data to filters is required in the future, it should be
done with some AVOptions-based system.

libavfilter/split.c

@@ -28,7 +28,7 @@
 #include "internal.h"
 #include "video.h"
 
-static int split_init(AVFilterContext *ctx, const char *args, void *opaque)
+static int split_init(AVFilterContext *ctx, const char *args)
 {
     int i, nb_outputs = 2;