scan and pcm audio fixes.

scan and pcm audio fixes.
 - lpcm audio fixed to handle 24 bit & interpret header correctly.
 - get aspect ratio from libmpeg2 rather than doing it ourselves.
 - announce when aspect ratio changes during preview scan.
 - if aspect ratio isn't either 4:3 or 16:9 complain & map to either 4:3 or 16:9 (whichever is closest).
 - start stream previews from file position 0 rather than 1/11 in case there's only on mpeg sequence header in the file.
 - don't give up on a file just because we can't get a preview due to a missing sequence header - only give up if we can't get any previews.
 - get audio bitstream characteristics during preview in a uniform way (we were treating PCM & MPEG audio specially which resulted in not getting their sample rate which caused a divide by zero in sync).

git-svn-id: svn://localhost/HandBrake/trunk@1370 b64f7644-9d1e-0410-96f1-a4d463321fa5