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/Graphics.h

@@ -252,6 +252,8 @@ public:
     void renderfixedpre(void);
     void renderfixedpost(void);
 
+    void draw_screenshot_border(void);
+
     bool Hitest(SDL_Surface* surface1, SDL_Point p1, SDL_Surface* surface2, SDL_Point p2);
 
     void drawentities(void);