UI: Always create a display capture on first run

For the sake of consistency, always create a display capture source on
the very first run of the program, just to have something displayed.

(NOTE: The only exception here is on windows 7/vista, which isn't ideal
for display capture, so it'll continue to be left blank)

obs/window-basic-main.hpp

@@ -113,6 +113,7 @@ private:
 
 	void          SetupEncoders();
 
+	void          CreateFirstRunSources(obs_scene_t *scene);
 	void          CreateDefaultScene();
 
 	void          ClearVolumeControls();