avcodec/dct-test: correct output bias of prores idct

This way the output values make sense
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dct-test.c

@@ -113,6 +113,10 @@ static void ff_prores_idct_put_10_sse2_wrap(int16_t *dst){
         tmp[i]= dst[i];
     }
     ff_prores_idct_put_10_sse2(dst, 16, tmp, qmat);
+
+    for(i=0; i<64; i++) {
+         dst[i] -= 512;
+    }
 }
 #endif