When != 0, this property will express the W:H ratio of the desired
output area (be it one monitor or the span of all monitors). At
the time of translating coordinates, coordinates will be skewed so
that the input area of the tablet is a rectangle of the given ratio.

Events happening in the input areas that fall outside the output
aspect ratio will be clamped to the nearest coordinate in the rect.

If the ratio is 0, the whole input area of the tablet will be used
and no coordinate skewing will happen.

https://bugzilla.gnome.org/show_bug.cgi?id=774115