dispatch: document some guarantees

The here documented guarantee might simplify code using this mechanism
a lot, because it becomes unnecessary to invent a separate mechanism to
make the mp_dispatch_queue_process loop exit after processing a dispatch
callback. (Instead, the dispatch callback can set a flag, and the caller
of mp_dispatch_queue_process can check it.)
demux_mkv: enable parsing for mp3

For some reason, some files appear to have broken mp3 packets, or at
least in a form that libavcodec can't deal with. The audio in the sample
file in question could not be decoded using libavcodec.

The problematic file had variable packet sizes, and the libavcodec
decoder kept printing "mp3: Header missing" for each packet it was fed.
Remuxing with mkvmerge fixes the problem. The mp3 data is probably not
VBR, and remuxing resulted in fixed-size mp3 frames. So I don't know why
the sample file was muxed this way - it might just be incorrect.

The sample file had "libmkv 0.6.4" as MuxingApp (although I could not
get mkvinfo to print this element, maybe the file uses an incorrect
element ID), and "HandBrake 0.9.4" as WritingApp.

Note that the libmpg123 decoder does not have any issues with it. It's
probably more robust, because libmpg123 was made to decode whole mp3
files, not just single frames.

Fixes issue #742.
af.rst: fix replaygain-track description

libav now supports reading ReplayGain values from LAME's Info/XING tag as well.
build: bump required libpostproc version

The CPU autodetect feature was added in 52.2.100 and is lacking from the
stand-alone version at http://git.videolan.org/?p=libpostproc.git