lavfi: add init_opaque callback

This will be used by filters which require an opaque field to be passed
in input. Should be required only for filters which imply a programmatic
use.

This is possibly a temporary solution, to be removed when we'll have a
clearer and better agreememnt of how/if to pass binary data for
initializing a filter.

See thread:
Subject: [FFmpeg-devel][PATCH] lavfi: add init2 callback
Date: Fri,  6 Jul 2012 01:22:17 +0200

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2011-04-18
 
 API changes, most recent first:
 
+2012-07-10 - xxxxxxx - lavfi 3.2.0 - avfilter.h
+  Add init_opaque() callback to AVFilter struct.
+
 2012-06-26 - xxxxxxx - lavu 51.63.100 - imgutils.h
   Add functions to libavutil/imgutils.h:
   av_image_get_buffer_size()