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Proper scaling for the mid folding

Nor scaling the mid only after we've been able to store it
for folding.

--- a/libcelt/bands.c

+++ b/libcelt/bands.c

@@ -249,6 +249,7 @@

    int j;

    celt_word32 xp=0, side=0;

    celt_word32 El, Er;

+   celt_word16 mid2;

 #ifdef FIXED_POINT

    int kl, kr;

 #endif

@@ -260,11 +261,12 @@

       xp = MAC16_16(xp, X[j], Y[j]);

       side = MAC16_16(side, Y[j], Y[j]);

    }

+   /* Compensating for the mid normalization */

+   xp = MULT16_32_Q15(mid, xp);

    /* mid and side are in Q15, not Q14 like X and Y */

-   mid = SHR32(mid, 1);

-   //side = SHR32(side, 1);

-   El = MULT16_16(mid, mid) + side - 2*xp;

-   Er = MULT16_16(mid, mid) + side + 2*xp;

+   mid2 = SHR32(mid, 1);

+   El = MULT16_16(mid2, mid2) + side - 2*xp;

+   Er = MULT16_16(mid2, mid2) + side + 2*xp;

    if (Er < EPSILON)

       Er = EPSILON;

    if (El < EPSILON)

@@ -289,7 +291,8 @@

    for (j=0;j<N;j++)

    {

       celt_norm r, l;

-      l = X[j];

+      /* Apply mid scaling (side is already scaled) */

+      l = MULT16_16_Q15(mid, X[j]);

       r = Y[j];

       X[j] = EXTRACT16(PSHR32(MULT16_16(lgain, SUB16(l,r)), kl+1));

       Y[j] = EXTRACT16(PSHR32(MULT16_16(rgain, ADD16(l,r)), kr+1));

@@ -865,9 +868,11 @@

          else

             next_level = level+1;

+         /* In stereo mode, we to not apply a scaling to the mid because we need the mormalized

+            mid for folding later */

          quant_band(encode, m, i, X, NULL, N, mbits, spread, B, intensity, tf_change,

                lowband, resynth, ec, remaining_bits, LM, next_lowband_out1,

-               NULL, next_level, seed, MULT16_16_P15(gain,mid), lowband_scratch, fill);

+               NULL, next_level, seed, stereo ? Q15ONE : MULT16_16_P15(gain,mid), lowband_scratch, fill);

          quant_band(encode, m, i, Y, NULL, N, sbits, spread, B, intensity, tf_change,

                next_lowband2, resynth, ec, remaining_bits, LM, NULL,

                NULL, next_level, seed, MULT16_16_P15(gain,side), NULL, fill && !stereo);