Show skip prompt during credits and ending picture

This adds a "- Press {button} to skip -" prompt to both the credits and
ending picture sequences.

It was always possible to skip them by pressing Enter, but not many
people knew this. In fact, even I didn't know this until I saw Elomavi
do it a year or so ago. So it's not really intuitive that this is
possible.

The prompt only shows up if you've completed the game before, and
disappears after two seconds similar to the "[Press {button} to return
to editor]" text.

Unfortunately, given how the game works, game completion is detected
based on if you have unlocked Flip Mode or not. At this point, the
unlock for the game being completed (unlock 5) will already be set to
true no matter what during the Plenary fanfare, but the Flip Mode unlock
(unlock 18) won't be until the player hits "play" on the main menu. As a
special case, the prompt will always show up in M&P (because Flip Mode
is always unlocked in M&P).

desktop_version/src/Game.h

@@ -531,6 +531,9 @@ public:
     bool inputdelay;
 
     bool statelocked;
+
+    int old_skip_message_timer;
+    int skip_message_timer;
 };
 
 #ifndef GAME_DEFINITION