Set backgrounddrawn to false when returning to pause menu

This makes it so the tower background doesn't persist and scroll upwards
if you exit the menu in a warp zone horizontal or vertical room.

Ugh, and while we're on the subject of separating the in-game tower
background and the menu tower background, could we PLEASE separate the
horizontal / vertical warp backgrounds from the tower backgrounds, too?!

desktop_version/src/Game.cpp

@@ -7763,6 +7763,7 @@ void Game::returntopausemenu()
     gamestate = MAPMODE;
     map.kludge_to_bg();
     map.tdrawback = true;
+    graphics.backgrounddrawn = false;
     game.mapheld = true;
     graphics.flipmode = graphics.setflipmode;
 }