increase the maximum AC3 probe buffer size and reduce the threshold for 50% reliability score to 2 frames.  Fixes out.ac3 from [FFmpeg-devel] AC3 Format Detection unreliable.
increase the maximum AC3 probe buffer size and reduce the threshold for 50% reliability score to 2 frames.  Fixes out.ac3 from [FFmpeg-devel] AC3 Format Detection unreliable.

git-svn-id: a4d7c1866f8397a4106e0b57fc4fbf792bbdaaaf@9920 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg