Touchpad: support natural scrolling for volume change (Scrolling Direction: Down motion scrolls up)

Important Information

Provide following Information:

• mpv version: https://github.com/stax76/mpv.net/releases/tag/5.4.8.0
• Windows Version: 10 x64 20H2
• Source of the mpv binary library: https://sourceforge.net/projects/mpv-player-windows/files/libmpv/mpv-dev-x86_64-20210228-git-d1be8bb.7z/download
• If known which version of mpv introduced the problem: This is a Feature request, so maybe this was missing from the beginning

Reproduction steps

Windows offers the touchpad setting "Scrolling Direction: Down motion scrolls up"

If this is enabled, scrolling up/down in mpv to change volume is wrong:

Expected behavior

• Motion "Scroll Down": should decrease the volume
• Motion "Scroll Up": should increase the volume

Actual behavior

• Motion "Scroll Down": actually increases the volume
• Motion "Scroll Up": actually decreases volume

Log file

I could not create this with mpv.net. I hope this is needed for my feature request.

Start by tracing how Windows touchpad scroll events reach volume adjustment in mpv, then compare that handling with the system's “Scrolling Direction: Down motion scrolls up” setting. Verify the change with scroll-up and scroll-down gestures while the setting is enabled; done means scroll down decreases volume and scroll up increases it.