Don't redraw H/V warp BG if gotorooming to same room in customs

This has two benefits:
 (1) The game uses less resources when it is asked to gotoroom to the
     same room because it is no longer redrawing the warp background
     every single frame, which is very wasteful.
 (2) The warp background no longer freezes or flickers if the player is
     standing inside a gotoroom script box (which calls gotoroom every
     frame or every other frame, because every time the gotoroom happens
     the script box gets reloaded).

desktop_version/src/Game.h

@@ -121,6 +121,7 @@ public:
     int door_up;
     int door_down;
     int roomx, roomy, roomchangedir;
+    int prevroomx, prevroomy;
     int temp, j, k;
 
     int savex, savey, saverx, savery;