Scale mouse sensitivity according to scaling

[Plagman]

So that perceived screen-space sensitivity is the same between eg. a game window and a smaller dialog that gets scaled up, etc.

Also need to scale mouse sensitivity in nested mode so screen-space pointer speed is similar in and out of the gamescope window, but that's more minor and for the future.

[Plagman]

We also quantize cursor positions on window-space coordinates right now, which can look bad at high scale ratios. Maybe we can virtualize that visually while we're at it, might be counterproductive, would have to test.