avfilter/vf_boxblur: avoid one addition per line

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_boxblur.c

@@ -221,11 +221,10 @@ static inline void blur(uint8_t *dst, int dst_step, const uint8_t *src, int src_
      */
     const int length = radius*2 + 1;
     const int inv = ((1<<16) + length/2)/length;
-    int x, sum = 0;
+    int x, sum = src[radius*src_step];
 
     for (x = 0; x < radius; x++)
         sum += src[x*src_step]<<1;
-    sum += src[radius*src_step];
 
     sum = sum*inv + (1<<15);