avcodec/mpegvideo: make mc_mb_var_sum_temp / mb_var_sum_temp 64bit

This avoids a hypothetical integer overflow with very high resolution video
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegvideo.h

@@ -180,8 +180,8 @@ typedef struct MotionEstContext{
     int stride;
     int uvstride;
     /* temp variables for picture complexity calculation */
-    int mc_mb_var_sum_temp;
-    int mb_var_sum_temp;
+    int64_t mc_mb_var_sum_temp;
+    int64_t mb_var_sum_temp;
     int scene_change_score;
 /*    cmp, chroma_cmp;*/
     op_pixels_func (*hpel_put)[4];