Set fademode to temp 0 when going to in-game options

While I've decoupled fademode from gamemode starting, being faded out on
the title screen results in a black screen and you being unable to make
any input. So we'll need to store the current fademode in a temporary
variable when going to in-game options, then put it back when we return
to the pause menu. Yes, you can turn on glitchrunner mode during the
in-game options, and then immediately return to the pause menu to
instantly go back to the title screen; this is intended.

Due to frame ordering, putting the fademode back needs to be deferred to
the end of the frame to prevent a 1-frame flicker.

It's actually sufficient enough to do this temporary fademode storage to
fix the whole thing, but I also decided to decouple fademode and
gamemode starting just to be sure.

desktop_version/src/Game.cpp

@@ -6666,6 +6666,11 @@ static void nextbgcolor(void)
     map.nexttowercolour();
 }
 
+static void setfademode(void)
+{
+    graphics.fademode = graphics.ingame_fademode;
+}
+
 void Game::returntoingame(void)
 {
     ingame_titlemode = false;
@@ -6684,6 +6689,7 @@ void Game::returntoingame(void)
         DEFER_CALLBACK(returntoingametemp);
         gamestate = MAPMODE;
         graphics.flipmode = graphics.setflipmode;
+        DEFER_CALLBACK(setfademode);
         if (!map.custommode && !graphics.flipmode)
         {
             obj.flags[73] = true;