Make AAC in Ogg (ogm) work.

This needs the extradata to be extracted.
The approach used is the one MPlayer uses, though it is
unclear whether the 4 bytes extradata that are skipped
should be skipped always or only for AAC.
The AAC parser must be disabled, too, otherwise playback
still does not work.
Fixes trac issue #547.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>