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/Input.cpp

@@ -2320,6 +2320,8 @@ static void mapmenuactionpress(void)
         game.gamestate = TITLEMODE;
         graphics.flipmode = false;
         game.ingame_titlemode = true;
+        graphics.ingame_fademode = graphics.fademode;
+        graphics.fademode = 0;
 
         // Set this before we create the menu
         game.kludge_ingametemp = game.currentmenuname;