Changed MPEG-2 resolution in the FFmpeg test.

Newer FFmpeg prohibits 4096x4096 MPEG-2, presumably because it violates
the standard.

http://git.videolan.org/gitweb.cgi/ffmpeg.git/?p=ffmpeg.git;a=commit;h=7fb87bc5f24b1be13269109506c05e4c54695b5e

modules/highgui/test/test_ffmpeg.cpp

@@ -104,7 +104,10 @@ public:
                 frame_s = Size(1920, 1080);*/
 
             if( tag == VideoWriter::fourcc('M', 'P', 'E', 'G') )
+            {
+                frame_s = Size(720, 576);
                 fps = 25;
+            }
 
             VideoWriter writer(filename, tag, fps, frame_s);