The window used NULL as a parent window, which defaults internally to
using the root window of the default screen. But at the time wintab is
initialized, there is no default display/screen yet.
Fix this by retrieving this information from the given GdkDeviceManager,
so we don't have to wait for the display to be in place before
initialization.
https://bugzilla.gnome.org/show_bug.cgi?id=743330