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.cpp

@@ -121,6 +121,8 @@ void VideoConfig::Refresh()
   bForceTrueColor = Config::Get(Config::GFX_ENHANCE_FORCE_TRUE_COLOR);
   bDisableCopyFilter = Config::Get(Config::GFX_ENHANCE_DISABLE_COPY_FILTER);
   bArbitraryMipmapDetection = Config::Get(Config::GFX_ENHANCE_ARBITRARY_MIPMAP_DETECTION);
+  fArbitraryMipmapDetectionThreshold =
+      Config::Get(Config::GFX_ENHANCE_ARBITRARY_MIPMAP_DETECTION_THRESHOLD);
 
   stereo_mode = Config::Get(Config::GFX_STEREO_MODE);
   iStereoDepth = Config::Get(Config::GFX_STEREO_DEPTH);