Feature Request: optional mouse residual decay via config

[qwenger]

Currently the mouse residual either decays exponentially each frame or remains constant, see https://github.com/mupen64plus/mupen64plus-input-sdl/blob/master/src/plugin.c#L656.

For FPS like "The World Is Not Enough" where aim is controlled with the stick and in turn with the mouse, this makes aiming highly difficult as the view continues rotating when one stops moving the mouse.

It would be nice to have a configuration setting to reset mousex_residual and mousey_residual to 0 instead of multiplying it by 224/256. Alternatively, the setting could control the integer factor (224 currently).