While adding stereo rematrixing, I came across something that needs to
be fixed even without adding the feature.  The output correctly uses 4
dummy values for the rematrixing flags in block-0, but the bit
allocation routine does not take these bits into account.  From what I
can tell, there was a patch in 2003 that corrected the output to make it
DVD and spec compatible, but it didn't correct the bit allocation.  It's
only 4 bits over the entire 6 blocks, so overflow errors would happen
rarely or never, but it's still worth fixing.  So here is a fix.

patch by (Justin Ruggles {jruggle earthlink net)

Originally committed as revision 4179 to svn://svn.ffmpeg.org/ffmpeg/trunk
