D3D: Move exclusive mode switching to UI thread.

This prevents deadlocks when switching to exclusive mode.
And it also allows the CPU thread to block until we've completed the switch.

Source/Core/VideoBackends/D3D/Render.h

@@ -25,6 +25,7 @@ public:
   void SetSamplerState(int stage, int texindex, bool custom_tex) override;
   void SetInterlacingMode() override;
   void SetViewport() override;
+  void SetFullscreen(bool enable_fullscreen) override;
 
   // TODO: Fix confusing names (see ResetAPIState and RestoreAPIState)
   void ApplyState(bool bUseDstAlpha) override;