[QOL] More precise mouse sensitivity for high DPI mouse players

[smidouni]

Suggestion

Right now we are only able to set mouse sensitivity in increments of 0.01 (even in the config file, it gets rounded on game launch).

I suggest instead being able to set increments of 0.0001 or even lower.

Reasons

High DPI (CPI) has proven lately to be viable due to recent mice having no smoothing at all on higher DPIs, and it is pretty much necessary to fully enjoy the benefits of recent high polling rate mice (8kHz) since the mouse sensor does not send events for small mouse movements on lower DPIs.

High DPI also reduces the mouse input lag by a few ms, providing a smoother experience for the player and potentially being useful for high AR plays.

This change would allow high DPI players like myself to set much more precise sensitivities so that they can experiment, and to avoid having to change mouse DPI when switching from another game to osu!.

It would also benefit regular mouse players on lower DPIs to some extent as well, especially those playing on 1600 DPI where a third digit might be useful for precision.