aacdec: Use macros for constants

This commit replaces the previous hardcoded constants with both new and previously
defined macros from aac.h. This change makes it easy for anyone reading the code
to know how encoding and decoding scalefactors works. It's also possibly
a step in unifying some of the code across both the encoder and decoder.

Reviewed-by: Claudio Freire <klaussfreire@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/aacdec.c

@@ -1394,7 +1394,7 @@ static int decode_scalefactors(AACContext *ac, float sf[120], GetBitContext *gb,
                                int band_type_run_end[120])
 {
     int g, i, idx = 0;
-    int offset[3] = { global_gain, global_gain - 90, 0 };
+    int offset[3] = { global_gain, global_gain - NOISE_OFFSET, 0 };
     int clipped_offset;
     int noise_flag = 1;
     for (g = 0; g < ics->num_window_groups; g++) {
@@ -1406,7 +1406,7 @@ static int decode_scalefactors(AACContext *ac, float sf[120], GetBitContext *gb,
             } else if ((band_type[idx] == INTENSITY_BT) ||
                        (band_type[idx] == INTENSITY_BT2)) {
                 for (; i < run_end; i++, idx++) {
-                    offset[2] += get_vlc2(gb, vlc_scalefactors.table, 7, 3) - 60;
+                    offset[2] += get_vlc2(gb, vlc_scalefactors.table, 7, 3) - SCALE_DIFF_ZERO;
                     clipped_offset = av_clip(offset[2], -155, 100);
                     if (offset[2] != clipped_offset) {
                         avpriv_request_sample(ac->avctx,
@@ -1419,9 +1419,9 @@ static int decode_scalefactors(AACContext *ac, float sf[120], GetBitContext *gb,
             } else if (band_type[idx] == NOISE_BT) {
                 for (; i < run_end; i++, idx++) {
                     if (noise_flag-- > 0)
-                        offset[1] += get_bits(gb, 9) - 256;
+                        offset[1] += get_bits(gb, NOISE_PRE_BITS) - NOISE_PRE;
                     else
-                        offset[1] += get_vlc2(gb, vlc_scalefactors.table, 7, 3) - 60;
+                        offset[1] += get_vlc2(gb, vlc_scalefactors.table, 7, 3) - SCALE_DIFF_ZERO;
                     clipped_offset = av_clip(offset[1], -100, 155);
                     if (offset[1] != clipped_offset) {
                         avpriv_request_sample(ac->avctx,
@@ -1433,7 +1433,7 @@ static int decode_scalefactors(AACContext *ac, float sf[120], GetBitContext *gb,
                 }
             } else {
                 for (; i < run_end; i++, idx++) {
-                    offset[0] += get_vlc2(gb, vlc_scalefactors.table, 7, 3) - 60;
+                    offset[0] += get_vlc2(gb, vlc_scalefactors.table, 7, 3) - SCALE_DIFF_ZERO;
                     if (offset[0] > 255U) {
                         av_log(ac->avctx, AV_LOG_ERROR,
                                "Scalefactor (%d) out of range.\n", offset[0]);