mpegvideo: Increase MAX_MV for HD video

Fixes out of array reads

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

libavcodec/mpegvideo.h

@@ -57,7 +57,7 @@ enum OutputFormat {
 #define QMAT_SHIFT 21
 
 #define MAX_FCODE 7
-#define MAX_MV 2048
+#define MAX_MV 4096
 
 #define MAX_THREADS 32
 #define MAX_PICTURE_COUNT 36