[FFmpeg-soc] ac3enc merge

[Justin Ruggles]

Hi,

I'm going to go ahead and start merging some of the AC3 encoder changes
I've been working on in my git tree.

For larger changes that might be controversial, I'll post patches on
ffmpeg-devel.  For example, the per-codec options for metadata and how
the floating-point/fixed-point split was handled.  Both of those could
be done in different ways; I just went with what I thought was the best
approach.  It would be good to get some input though before committing
them to SVN.  Plus there are some regression test and configure changes
in there that need review.

Cheers,
Justin