If the wl_surface receiving touch events is destroyed, we will get no
wl_touch.up event to remove the touchpoint from our internal accounting.
Check for this, and drop touchpoints happening in surfaces that do
disappear during operation.
Also update the cursor surfaces of every seat when an output changes
scale. This could for example happen when a monitor scale is changed via
Settings.
GdkWaylandDeviceData conceptually gathers the data that belongs to
a seat, so it's been renamed (although the old typedef stays, plenty
of refactoring is due here...).
The methods in GdkSeatClass have also been implemented, the most
remarkable is ::grab, which ensures the grab is performed on all
the relevant "master" devices.
https://bugzilla.gnome.org/show_bug.cgi?id=759309
