Normalise scroll values
Created by: ascent12
Fixes #848 (closed)
Apparently Xwayland will accumulate the scroll amount between events, and each 10 "units" of scroll send one X11 scroll event. Since libinput was sending 15 units per mouse scroll for me, 2 actual scrolls would end up as 3 X11 scrolls.
You can see the weston fix here: https://github.com/wayland-project/weston/blob/master/libweston/libinput-device.c#L184
I also took the liberty of cleaning up the libinput axis function a bit.