j2k: change fixed point of stepsize to 16.16

This seems more natural and matches jpeg2000
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/j2k.c

@@ -289,7 +289,7 @@ int ff_j2k_init_component(Jpeg2000Component *comp,
             }
             /* BITEXACT computing case --> convert to int */
 //             if (avctx->flags & CODEC_FLAG_BITEXACT)
-            band->stepsize = stepsize * (1 << 13);
+            band->stepsize = stepsize * (1 << 16);
 
             /* computation of tbx_0, tbx_1, tby_0, tby_1
              * see ISO/IEC 15444-1:2002 B.5 eq. B-15 and tbl B.1

libavcodec/j2kdec.c

@@ -830,7 +830,7 @@ static int decode_tile(Jpeg2000DecoderContext *s, Jpeg2000Tile *tile)
                                 int *datap = &comp->data[(comp->coord[0][1] - comp->coord[0][0]) * (y+j) + x];
                                 int *ptr = t1.data[j];
                                 for (i = 0; i < (cblk->coord[0][1] - cblk->coord[0][0]); ++i) {
-                                    int tmp = ((int64_t)ptr[i]) * ((int64_t)band->stepsize) >> 13, tmp2;
+                                    int tmp = ((int64_t)ptr[i]) * ((int64_t)band->stepsize) >> 16, tmp2;
                                     tmp2 = FFABS(tmp>>1) + (tmp&1);
                                     datap[i] = tmp < 0 ? -tmp2 : tmp2;
                                 }

libavcodec/j2kenc.c

@@ -802,7 +802,7 @@ static void truncpasses(Jpeg2000EncoderContext *s, Jpeg2000Tile *tile)
                         Jpeg2000Cblk *cblk = prec->cblk + cblkno;
 
                         cblk->ninclpasses = getcut(cblk, s->lambda,
-                                (int64_t)dwt_norms[codsty->transform][bandpos][lev] * (int64_t)band->stepsize >> 13);
+                                (int64_t)dwt_norms[codsty->transform][bandpos][lev] * (int64_t)band->stepsize >> 16);
                     }
                 }
             }
@@ -863,7 +863,7 @@ static int encode_tile(Jpeg2000EncoderContext *s, Jpeg2000Tile *tile, int tileno
                                 int *ptr = t1.data[y-yy0];
                                 for (x = xx0; x < xx1; x++){
                                     *ptr = (comp->data[(comp->coord[0][1] - comp->coord[0][0]) * y + x]);
-                                    *ptr = (int64_t)*ptr * (int64_t)(8192 * 8192 / band->stepsize) >> 13 - NMSEDEC_FRACBITS;
+                                    *ptr = (int64_t)*ptr * (int64_t)(16384 * 65536 / band->stepsize) >> 14 - NMSEDEC_FRACBITS;
                                     ptr++;
                                 }
                             }