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().

libobs/obs.c

@@ -1014,7 +1014,7 @@ int obs_reset_video(struct obs_video_info *ovi)
 	if (!obs) return OBS_VIDEO_FAIL;
 
 	/* don't allow changing of video settings if active. */
-	if (obs->video.video && video_output_active(obs->video.video))
+	if (obs->video.video && obs_video_active())
 		return OBS_VIDEO_CURRENTLY_ACTIVE;
 
 	if (!size_valid(ovi->output_width, ovi->output_height) ||
@@ -2258,3 +2258,12 @@ obs_data_t *obs_get_private_data(void)
 	obs_data_addref(private_data);
 	return private_data;
 }
+
+bool obs_video_active(void)
+{
+	struct obs_core_video *video = &obs->video;
+	if (!obs)
+		return false;
+
+	return os_atomic_load_long(&video->raw_active) > 0;
+}