[Feature request] Override hardcoded game input deadzone

[dsg22]

A lot of games have a hardcoded, sometimes huge, input deadzone for controllers. It would be great if Special K could work around this by adding a user-configured amount to the XInput stick values returned to the game.

Steam has a good implementation for this, it allows configuring two things:

• The game deadzone to null out as above
• A "deadzone buffer" which effectively becomes the new deadzone. Values > game_deadzone but < (game_deadzone + deadzone_buffer) return 0.