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.cpp

@@ -364,6 +364,10 @@ void Game::init(void)
     old_mode_indicator_timer = 0;
     mode_indicator_timer = 0;
 
+    old_screenshot_border_timer = 0;
+    screenshot_border_timer = 0;
+    screenshot_saved_success = false;
+
     setdefaultcontrollerbuttons();
 }