Frame: Use PauseAndLock when switching fullscreen modes.

This allows us to regain exclusive mode directly from OnActive().

Source/Core/VideoBackends/D3D/Render.h

@@ -26,6 +26,7 @@ public:
   void SetInterlacingMode() override;
   void SetViewport() override;
   void SetFullscreen(bool enable_fullscreen) override;
+  bool IsFullscreen() override;
 
   // TODO: Fix confusing names (see ResetAPIState and RestoreAPIState)
   void ApplyState(bool bUseDstAlpha) override;