libobs-d3d11: Clear device state before unloading

Unloads all device data and clears all device references.  Probably not
necessary, but it's unknown how D3D11 handles this internally so
probably best to be safe.

libobs-d3d11/d3d11-subsystem.hpp

@@ -679,4 +679,5 @@ struct gs_device {
 	void UpdateViewProjMatrix();
 
 	gs_device(uint32_t adapterIdx);
+	~gs_device();
 };