Fixes #4537. I had hoped the input count validation would work globally, but apparently it regresses that particular game.

There's no nicer way to handle this unfortunately, since we're dealing with entirely undocumented behavior, so I've hidden the offending validation behind a config option and enabled it only for The Void, the only known game that actually needs it.

Also part of this PR, I've refactored the code to sit in its own .cpp file (as I should have done from the start really).