Sundry tweaks to aos/vision libs

Change-Id: Ia5578dcf2d42ac53b81af239bf329eb084fcf1d9
diff --git a/aos/vision/debug/debug_viewer.h b/aos/vision/debug/debug_viewer.h
index 3bada28..ab2c716 100644
--- a/aos/vision/debug/debug_viewer.h
+++ b/aos/vision/debug/debug_viewer.h
@@ -66,6 +66,7 @@
   std::function<void(uint32_t)> key_press_event;
 
  private:
+  bool shown_yet_ = false;
   double scale_factor = 1.0;
   int window_width_ = 100;
   int window_height_ = 100;