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