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.cpp

@@ -391,6 +391,7 @@ void Game::init(void)
 
     ingame_titlemode = false;
     kludge_ingametemp = Menu::mainmenu;
+    shouldreturntopausemenu = false;
 
     /* Terry's Patrons... */
     const char* superpatrons_arr[] = {
@@ -7766,4 +7767,5 @@ void Game::returntopausemenu()
     graphics.backgrounddrawn = false;
     game.mapheld = true;
     graphics.flipmode = graphics.setflipmode;
+    game.shouldreturntopausemenu = true;
 }