Ensure that newly registered video displays also have a frame present so they can actually render.

(This causes a problem with detached video on wxGTK but not on wxMSW, oddly.)

Originally committed to SVN as r5439.

aegisub/src/video_context.cpp

@@ -333,6 +333,7 @@ void VideoContext::AddDisplay(VideoDisplay *display) {
 		if ((*cur) == display) return;
 	}
 	displayList.push_back(display);
+	UpdateDisplays(true);
 }