fix audio detection problem during scan of ffmpeg audio sources

fix audio detection problem during scan of ffmpeg audio sources
audio frames can be larger than their container packet sizes, but during
scan, we only feed one container packet to the decoder, then reset
the decoder and try the next packet.  so the audio is never detected.

as buffers are tested, they are added to a cache.  the entire cache is
passed to the decoder to scan for info.  the cache is limited to 4KB.

git-svn-id: svn://localhost/HandBrake/trunk@3195 b64f7644-9d1e-0410-96f1-a4d463321fa5