mp_image: provide function to convert mp_image to AVFrame
vf_lavfi: add libavfilter bridge

Requires recent Libav git versions. Earlier versions will not be
supported, as the API is different.

Note that this is sort of useless, because the option parser prevents
you from making use of the full libavfilter graph syntax. This has to be
fixed somehow.

Most of the filter creation code (most of the config() function) has
been taken from avplay.c.

The code is completely different from MPlayer vf_lavfi, which doesn't
compile as it hasn't been updated through multiple libavfilter API
changes.