Parse the bitrate field in the ASMRuleBook ("AverageBandwidth") to fill in

Parse the bitrate field in the ASMRuleBook ("AverageBandwidth") to fill in
the AVStream->AVCodecContext->bit_rate field, which is not in the MDPR block
(the "OpaqueData" SDP field). This allows clients to choose streams based
on their bitrate, which is what most network-players base stream selection
on. (Of course, it is also possible to select based on anything else, that
is entirely up to the client.) See "[PATCH] rdt.c: ASM rulebook bitrate
reading" thread on mailinglist.

Originally committed as revision 16467 to svn://svn.ffmpeg.org/ffmpeg/trunk