Cvar::Callback works inconsistently for gamelogic cvars

[slipher]

If you leave a Cvar::Callback cvar in the gamelogic at its default value, then the callback is not called the first time the gamelogic loads, but it is called on the second gamelogic load. Apparently since a value is present in the engine's cvar map after the first load, this triggers a change event when the cvar is registered on the second load.