avcodec/jpeg2000dec: Add 5/3 de-quantization special case for the lossless case

This avoids a multiplication and 64bits
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/jpeg2000dec.c

@@ -1436,8 +1436,14 @@ static void dequantization_int(int x, int y, Jpeg2000Cblk *cblk,
     for (j = 0; j < (cblk->coord[1][1] - cblk->coord[1][0]); ++j) {
         int32_t *datap = &comp->i_data[(comp->coord[0][1] - comp->coord[0][0]) * (y + j) + x];
         int *src = t1->data[j];
-        for (i = 0; i < w; ++i)
-            datap[i] = (src[i] * (int64_t)band->i_stepsize) / 32768;
+        if (band->i_stepsize == 16384) {
+            for (i = 0; i < w; ++i)
+                datap[i] = src[i] / 2;
+        } else {
+            // This should be VERY uncommon
+            for (i = 0; i < w; ++i)
+                datap[i] = (src[i] * (int64_t)band->i_stepsize) / 32768;
+        }
     }
 }