Add option to re-enable 1-frame input delay

This is an option for speedrunners whose muscle memory is precisely
trained and used to the 1-frame input delay that existed in 2.2 and
below. It is located in Game Options -> Advanced Options, and is off by
default.

To re-add the 1-frame input delay, we simply move the key.Poll() to the
start of the frame, instead of before an input function gets ran -
undoing what #535 did.

There is a frame ordering-sensitive issue here, where toggling
game.inputdelay at the wrong time could cause double-polling. However,
we only toggle it in an input function, which regardless is always
guaranteed to be ran after key.Poll() (it either happened at the start
of the frame or just before the input function got ran), so this is not
an issue. But, in case we ever need to toggle this variable in the
future, we can just use the defer callbacks system to defer the toggle
to the end of the frame - also added by #535.

Added at the request of Habeechee on the VVVVVV speedrunning Discord
server.

desktop_version/src/Game.h

@@ -452,6 +452,7 @@ public:
     void unlockAchievement(const char *name);
 
     bool disablepause;
+    bool inputdelay;
 };
 
 #ifndef GAME_DEFINITION