Fix deltaframe tower BG flicker when exiting menu in H/V warp room

To fix this annoying flicker (which, btw, took me WAY too long to do), I
had to introduce yet another kludge variable to signal that the
horizontal/vertical warp background should be re-initialized on the
pause screen.

I think I could technically keep the 'graphics.backgrounddrawn = false;'
in maplogic() and remove the 'graphics.backgrounddrawn = false;' in
Game::returntopausemenu(), but I'm keeping that other one around because
it doesn't hurt and just as a general precaution and safety measure.

desktop_version/src/Game.h

@@ -408,6 +408,7 @@ public:
 
     bool ingame_titlemode;
 
+    bool shouldreturntopausemenu;
     void returntopausemenu();
 };