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Fix collapse mask tracking for recombine steps.

The recombine loop for cm was correct if one started at 1 block,
 but was wrong otherwise (for a test case, convert 2 recombined
 blocks back to 4 with an initial cm of 0x3; the result should be
 0xF, but instead you get 0x7).
The recombine loop for fill was always wrong (for a test case,
 combine 8 blocks down to 1 with an initial fill=0xFE; the low bit
 remains unset).
This now properly interleaves and deinterleaves bits for these
 steps, which avoids declaring collapses (and skipping folding)
 where none, in fact, occurred.

--- a/libcelt/bands.c

+++ b/libcelt/bands.c

@@ -705,11 +705,14 @@

       for (k=0;k<recombine;k++)

       {

+         static const unsigned char bit_interleave_table[16]={

+           0,1,1,1,2,3,3,3,2,3,3,3,2,3,3,3

+         };

          if (encode)

             haar1(X, N>>k, 1<<k);

          if (lowband)

             haar1(lowband, N>>k, 1<<k);

-         fill |= fill<<(1<<k);

+         fill = bit_interleave_table[fill&0xF]|bit_interleave_table[fill>>4]<<2;

       }

       B>>=recombine;

       N_B<<=recombine;

@@ -1126,7 +1129,11 @@

          for (k=0;k<recombine;k++)

          {

-            cm |= cm<<(1<<k);

+            static const unsigned char bit_deinterleave_table[16]={

+              0x00,0x03,0x0C,0x0F,0x30,0x33,0x3C,0x3F,

+              0xC0,0xC3,0xCC,0xCF,0xF0,0xF3,0xFC,0xFF

+            };

+            cm = bit_deinterleave_table[cm];

             haar1(X, N0>>k, 1<<k);

          }

          B<<=recombine;