get rid of idiotc double subtraction

Originally committed as revision 10154 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/snow.c

@@ -775,7 +775,7 @@ static av_always_inline void liftS(DWTELEM *dst, DWTELEM *src, DWTELEM *ref, int
     int i;
 
     assert(shift == 4);
-#define LIFTS(src, ref, inv) ((inv) ? (src) - (((ref) - 4*(src))>>shift): (16*4*(src) + 4*(ref) + 8 + (5<<27))/(5*16) - (1<<23))
+#define LIFTS(src, ref, inv) ((inv) ? (src) + (((ref) + 4*(src))>>shift): (16*4*(src) + 4*(ref) + 8 + (5<<27))/(5*16) - (1<<23))
     if(mirror_left){
         dst[0] = LIFTS(src[0], mul*2*ref[0]+add, inverse);
         dst += dst_step;
@@ -1344,7 +1344,7 @@ void ff_snow_horizontal_compose97i(DWTELEM *b, int width){
 
     lift (temp   , b      , b   +w2, 1, 1, 1, width,  W_DM, W_DO, W_DS, 0, 1);
     lift5(temp+w2, b   +w2, temp   , 1, 1, 1, width,  W_CM, W_CO, W_CS, 1, 1);
-    liftS(b      , temp   , temp+w2, 2, 1, 1, width, -W_BM, W_BO, W_BS, 0, 1);
+    liftS(b      , temp   , temp+w2, 2, 1, 1, width,  W_BM, W_BO-1, W_BS, 0, 1);
     lift (b+1    , temp+w2, b      , 2, 1, 2, width, -W_AM, W_AO, W_AS, 1, 1);
 }