input devices: better touchpad support (in particular, on MacOS)

[jwise]

On MacOS, a common idiom is to pinch to zoom on a touchpad, and use two fingers for X-Y scroll. This works well in KiCad, for instance. In ngscopeclient, pinching to zoom does not do anything in particular, and combined X-Y scroll does something completely chaotic. (It is a common idiom in macOS apps to mute one axis when the other axis has high amplitude, or by some kind of heuristic, or something, to constrain to either X or Y scrolling; it is very difficult to precisely position when ngscopeclient is both zooming and scrolling!) It would be better, on a touchpad, for Y-scroll to not function as zoom, and X-scroll to function as linear progressive pan with very limited acceleration (if any); two finger pinch would then function as zoom.