Make the arbitrary mipmap detection threshold configureable

This is likely a "superuser" option at best, but I want to be able to play with
it without rebuilding if I want to tweak the heuristics

Source/Core/VideoCommon/VideoConfig.h

@@ -75,6 +75,7 @@ struct VideoConfig final
   bool bForceTrueColor;
   bool bDisableCopyFilter;
   bool bArbitraryMipmapDetection;
+  float fArbitraryMipmapDetectionThreshold;
 
   // Information
   bool bShowFPS;