Add border to indicate taking screenshot

Originally, I was thinking of adding a notification text that you took a
screenshot, but this is better because it is language-agnostic and it
doesn't contribute to potential UI clutter/clashing.

It flashes yellow if the screenshot successfully saved, and red if it
didn't.

desktop_version/src/Game.h

@@ -592,6 +592,10 @@ public:
 
     int old_mode_indicator_timer;
     int mode_indicator_timer;
+
+    int old_screenshot_border_timer;
+    int screenshot_border_timer;
+    bool screenshot_saved_success;
 };
 
 #ifndef GAME_DEFINITION