VideoCommon: Limit maximum denominator for MPEG4

It happened to be under the limit normally, but now that the VBI rate can be changed, that's no longer the case.

Source/Core/VideoCommon/FrameDumpFFMpeg.h

@@ -4,6 +4,7 @@
 #pragma once
 
 #include <ctime>
+#include <limits>
 #include <memory>
 
 #include "Common/CommonTypes.h"
@@ -62,6 +63,9 @@ private:
   // Used for filename generation.
   std::time_t m_start_time = {};
   u32 m_file_index = 0;
+
+  // Some codecs (like MPEG4) have a limit to this
+  int64_t m_max_denominator = std::numeric_limits<s64>::max();
 };
 
 #if !defined(HAVE_FFMPEG)