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

libavcodec/ac3enc.c

@@ -703,8 +703,10 @@ static int compute_bit_allocation(AC3EncodeContext *s,
     /* audio blocks */
     for(i=0;i<NB_BLOCKS;i++) {
         frame_bits += s->nb_channels * 2 + 2; /* blksw * c, dithflag * c, dynrnge, cplstre */
-        if (s->acmod == 2)
+        if (s->acmod == 2) {
             frame_bits++; /* rematstr */
+            if(i==0) frame_bits += 4;
+        }
         frame_bits += 2 * s->nb_channels; /* chexpstr[2] * c */
 	if (s->lfe)
 	    frame_bits++; /* lfeexpstr */