avfilter/vf_deshake: fix loss of precission with odd resolutions

Fixes part of Ticket3466
Found-by: Andrey_Karpov / PVS-Studio
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_deshake.c

@@ -306,8 +306,8 @@ static void find_motion(DeshakeContext *deshake, uint8_t *src1, uint8_t *src2,
         //av_log(NULL, AV_LOG_ERROR, "\n");
     }
 
-    p_x = (center_x - width / 2);
-    p_y = (center_y - height / 2);
+    p_x = (center_x - width / 2.0);
+    p_y = (center_y - height / 2.0);
     t->vector.x += (cos(t->angle)-1)*p_x  - sin(t->angle)*p_y;
     t->vector.y += sin(t->angle)*p_x  + (cos(t->angle)-1)*p_y;