This differs from the MPEG specification as the actual real world
files do compute their CRC over variable areas and not the fixed
ones listed in the specification. This is also the reason for
the complexity of this code and the need to perform the CRC
check for layer2 in the middle of layer2 decoding.
Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>