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/KeyPoll.cpp

@@ -186,7 +186,10 @@ void KeyPoll::Poll(void)
 
             if (evt.key.keysym.sym == SDLK_F6 && !evt.key.repeat)
             {
-                SaveScreenshot();
+                const bool success = SaveScreenshot();
+                game.old_screenshot_border_timer = 255;
+                game.screenshot_border_timer = 255;
+                game.screenshot_saved_success = success;
             }
 
             BUTTONGLYPHS_keyboard_set_active(true);