libobs: Add flush to video pipeline

On certain GPUs, if you don't flush and the window is minimized it can
endlessly accumulate memory due to what I'm assuming are driver design
flaws (though I can't know for sure).  The flush seems to prevent this
from happening, at least from my tests.  It would be nice if this
weren't necessary.

libobs/obs-video.c

@@ -424,6 +424,8 @@ static inline void output_frame(uint64_t timestamp)
 	render_video(video, cur_texture, prev_texture, timestamp);
 	frame_ready = download_frame(video, prev_texture, &frame);
 
+	gs_flush();
+
 	gs_leave_context();
 
 	if (frame_ready) {