lavfi/delogo: don't recompute the same difference again and again

The top left hand corner pixel coordinates are already stored in
logo_x1 and logo_y1 so don't recompute each of them 6 times for every
iteration.

This is a simple code optimization, result is obviously the same. The
performance gain is small (about 2% in my tests) but still good to
have, and the new code is clearer.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Reviewed-by; Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_delogo.c

@@ -100,21 +100,21 @@ static void apply_delogo(uint8_t *dst, int dst_linesize,
             weightb = (uint64_t)(x-logo_x1) * (logo_x2-1-x) * (y-logo_y1)                 * sar.num;
 
             interp =
-                (topleft[src_linesize*(y-logo_y  -yclipt)]   +
-                 topleft[src_linesize*(y-logo_y-1-yclipt)]   +
-                 topleft[src_linesize*(y-logo_y+1-yclipt)])  * weightl
+                (topleft[src_linesize*(y-logo_y1)]    +
+                 topleft[src_linesize*(y-logo_y1-1)]  +
+                 topleft[src_linesize*(y-logo_y1+1)]) * weightl
                 +
-                (topright[src_linesize*(y-logo_y-yclipt)]    +
-                 topright[src_linesize*(y-logo_y-1-yclipt)]  +
-                 topright[src_linesize*(y-logo_y+1-yclipt)]) * weightr
+                (topright[src_linesize*(y-logo_y1)]    +
+                 topright[src_linesize*(y-logo_y1-1)]  +
+                 topright[src_linesize*(y-logo_y1+1)]) * weightr
                 +
-                (topleft[x-logo_x-xclipl]    +
-                 topleft[x-logo_x-1-xclipl]  +
-                 topleft[x-logo_x+1-xclipl]) * weightt
+                (topleft[x-logo_x1]    +
+                 topleft[x-logo_x1-1]  +
+                 topleft[x-logo_x1+1]) * weightt
                 +
-                (botleft[x-logo_x-xclipl]    +
-                 botleft[x-logo_x-1-xclipl]  +
-                 botleft[x-logo_x+1-xclipl]) * weightb;
+                (botleft[x-logo_x1]    +
+                 botleft[x-logo_x1-1]  +
+                 botleft[x-logo_x1+1]) * weightb;
             interp /= (weightl + weightr + weightt + weightb) * 3U;
 
             if (y >= logo_y+band && y < logo_y+logo_h-band &&