win-capture: Fix bug with GDI monitor capture

With the new code that checks to see if the source is visible, I didn't
realize that I actually didn't set the source variable, so it would end
up never actually drawing.

plugins/win-capture/monitor-capture.c

@@ -132,10 +132,10 @@ static void *monitor_capture_create(obs_data_t *settings, obs_source_t *source)
 
 	capture = bzalloc(sizeof(struct monitor_capture));
 	capture->opaque_effect = opaque_effect;
+	capture->source = source;
 
 	update_settings(capture, settings);
 
-	UNUSED_PARAMETER(source);
 	return capture;
 }