[client] overlay: let the overlay know if it's in interactive mode

This also removes the need for the flags member as the overlay can just
opt to not render if it's not in interactive mode.

client/src/app.c

@@ -752,7 +752,8 @@ int app_renderOverlay(struct Rect * rects, int maxRects)
       ll_walk(g_state.overlays, (void **)&overlay); )
   {
     const int written =
-      overlay->ops->render(overlay->udata, false, buffer, MAX_OVERLAY_RECTS);
+      overlay->ops->render(overlay->udata, g_state.overlayInput,
+          buffer, MAX_OVERLAY_RECTS);
 
     for (int i = 0; i < written; ++i)
     {