Add asserts if ingame_titlemode in unexpected places

The game will now assert if the main menu is created while
ingame_titlemode is true, or if we attempt to load into a mode while
it's true. And if assertions are disabled then it just stops doing it
anyway.

I don't think there's any way to get a glitched ingame_titlemode again,
ever since I removed save data deletion taking you back to the main
menu. But I've had enough bugs with the fact that we more-or-less use
the same state for main menu options and in-game options, and that
glitched ingame_titlemode bug DID just happen, so I'm taking
precautions.

desktop_version/src/Input.cpp

@@ -290,6 +290,13 @@ static void startmode(const int mode)
 
 static void handlefadetomode(void)
 {
+    if (game.ingame_titlemode)
+    {
+        /* We shouldn't be here! */
+        SDL_assert(0 && "Loading a mode from in-game options!");
+        return;
+    }
+
     if (fadetomodedelay > 0)
     {
         --fadetomodedelay;