libobs: Add gs_begin_frame for duplicators

We really shouldn't be resetting duplicator state as part of gs_flush.
gs_begin_scene is not ideal because it is called twice per frame, and
only after duplicators have been ticked. Even though it makes no
user-facing difference, it makes more logical sense to reset at the top
of the frame than the bottom.

libobs/obs-video.c

@@ -866,6 +866,10 @@ void *obs_graphics_thread(void *param)
 
 		profile_start(video_thread_name);
 
+		gs_enter_context(obs->video.graphics);
+		gs_begin_frame();
+		gs_leave_context();
+
 		profile_start(tick_sources_name);
 		last_time = tick_sources(obs->video.video_time, last_time);
 		profile_end(tick_sources_name);