libobs: Add obs_video_active() function

(This commit also modifies UI)

Adds a universal function for determining whether video output is
currently active, rather than having to use video_output_active() on the
value returned by obs_get_video().

UI/win-update/win-update.cpp

@@ -586,7 +586,7 @@ try {
 
 	auto ActiveOrGameCaptureLocked = [this] ()
 	{
-		if (video_output_active(obs_get_video())) {
+		if (obs_video_active()) {
 			if (manualUpdate)
 				info(QTStr("Updater.Running.Title"),
 				     QTStr("Updater.Running.Text"));