Support more atrac3-in-mkv samples.

The mkv demuxer sometimes finds 12 bytes of realmedia
extradata after the matroska real audio properties.
pcx: fix rounding in bytes_per_scanline check

Fixes out of array reads

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>