Disable Texture Filtering dropdown when Manual Texture Sampling is checked

See https://bugs.dolphin-emu.org/issues/13049

Source/Core/DolphinQt/Config/Graphics/GraphicsWindow.h

@@ -24,6 +24,7 @@ public:
 
 signals:
   void BackendChanged(const QString& backend);
+  void UseFastTextureSamplingChanged();
 
 private:
   void CreateMainLayout();