audio: use AVBufferRef to allocate audio frames

A first step towards refcounted audio frames.

Amazingly, the API just does what we want, and the code becomes
simpler. We will need to NIH allocation from a pool, though.
dvd, bluray: reload demuxer on title changes

Causes the player to reload the demuxer and to relist the found
streams. Probably slightly dangerous/broken, because the demuxer
thread and possibly even the decoders will keep reading data from
the new title before the new demuxer takes over.

Fixes #1250.