De-duplicate menu animation code when bringing up map screen

When bringing up the map screen, the game does a small menu animation
where the menu comes in from the bottom. The code to calculate the menu
offset is copy-pasted everywhere, so I thought I'd de-duplicate it to
make my life easier when working with it. I also included the
game.gamestate assignment in the de-duplicated function, so it would be
easier for a future bugfix.

At the same time, I'm also removing all the BlitSurfaceStandard()s that
copied menubuffer to backBuffer. The red flag is that this blit happened
for every single entry point to MAPMODE and TELEPORTERMODE, except for
the script command gamemode(teleporter). Pressing Enter to bring up the
map screen, pressing Enter to quit the Super Gravitron, pressing Esc to
bring up the pause screen, and pressing Enter to bring up the teleporter
screen all do this blit, so if this blit was there to fix a bug, then
there's a bug with using the script command gamemode(teleporter)... but,
as far as I can tell, there isn't.

That's because the blit basically does nothing. All the blit does is
copy menubuffer onto backBuffer. Then the next thing that happens is
that either maprender() or teleporterrender() will be called, and the
first thing that those functions will always do is fill backBuffer with
solid black, completely overriding the previous blit. So that's why
removing this blit won't have any effect, and it can be safely removed
for code clarity.

desktop_version/src/Game.cpp

@@ -7146,3 +7146,16 @@ void Game::unlockAchievement(const char *name) {
     if (!map.custommode) NETWORK_unlockAchievement(name);
 #endif
 }
+
+void Game::mapmenuchange(const int newgamestate)
+{
+    gamestate = newgamestate;
+    graphics.resumegamemode = false;
+
+    graphics.menuoffset = 240;
+    if (map.extrarow)
+    {
+        graphics.menuoffset -= 10;
+    }
+    graphics.oldmenuoffset = graphics.menuoffset;
+}