libobs: Support device loss registration

Complex external systems using the D3D11 device may need to perform
their own device loss handling, the upcoming Windows Graphics Capture
support for example.

libobs-d3d11/d3d11-subsystem.hpp

@@ -946,6 +946,7 @@ struct gs_device {
 	matrix4 curViewMatrix;
 	matrix4 curViewProjMatrix;
 
+	vector<gs_device_loss> loss_callbacks;
 	gs_obj *first_obj = nullptr;
 
 	void InitCompiler();