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Adds spreading to the surround masking computation

This avoids a single low-energy band from changing the masking results.

--- a/celt/celt_encoder.c

+++ b/celt/celt_encoder.c

@@ -1411,13 +1411,23 @@

          st->energy_save[i]=bandLogE[i]-offset;

       st->energy_save=NULL;

    }

+   /* This computes how much masking takes place between surround channels */

    if (st->energy_mask&&!st->lfe)

    {

       opus_val32 mask_avg=0;

       opus_val16 offset = shortBlocks?HALF16(SHL16(LM, DB_SHIFT)):0;

       for (c=0;c<C;c++)

+      {

+         opus_val16 followE, followMask;

+         followE = followMask = -QCONST16(14.f, DB_SHIFT);

          for(i=0;i<st->end;i++)

-            mask_avg += bandLogE[nbEBands*c+i]-offset-st->energy_mask[nbEBands*c+i];

+         {

+            /* We use a simple follower to approximate the masking spreading function. */

+            followE = MAX16(followE-QCONST16(1.f, DB_SHIFT), bandLogE[nbEBands*c+i]-offset);

+            followMask = MAX16(followMask-QCONST16(1.f, DB_SHIFT), st->energy_mask[nbEBands*c+i]);

+            mask_avg += followE-followMask;

+         }

+      }

       surround_masking = DIV32_16(mask_avg,C*st->end) + QCONST16(.0f, DB_SHIFT);

       surround_masking = MIN16(MAX16(surround_masking,-QCONST16(1.5f, DB_SHIFT)), 0);

    }