emulate_discrete_scroll > 0 can invert scroll direction

[Janrupf]

When the (default) input:emulate_discrete_scroll is set to 1 or higher, scrolling up with the mouse causes Windows in the VM accessed by looking glass to scroll up (looking glass works over spice and just forwards the events, so I think while the issue shows up here, its not specific to looking glass).

This happens with my MX Master 3, but I assume any mouse that can emit fine grained scroll events could cause this.

There was a PR to work around this in looking glass: gnif/LookingGlass#1146 , but it ultimately got rejected.