[mythtv] FFmpeg sync for 0.24

[Janne Grunau]

Hi,

http://git.jannau.net/git/mythtv.complete/log/?h=ffmpeg_sync_25753 has
the FFmpeg sync for 0.24 and a couple of cleanup patches. It's based on
our changeset 25753. It would make my life easier if no changes to
libs/libav*, libs/libswscale and libs/libpostproc are committed while
this is pending.

There are 2 reasons why it's only available via my git tree. The patch
is huge (5/36M compressed/uncompressed) and will probably mess up svn
checkouts due to the move of libav* to external/FFmpeg.