We now have a proper MASTER/SLAVE input device split, where
the masters are virtual core input devices and we add fake hw
slave devices for the system pointer and real slave devices for
wintab devices.
We also set the proper source_device on the events so you can
tell which device sent it and properly decode the axis info.
There are sure regressions but basic stuff seems to be working
again after all the API breakage done with comments like
"Win32 and Quartz need to be ported still."
This function makes a better replacement for
gdk_display_get_core_pointer(), wherever it might yet be needed, for
XI2 resorts to XIGetClientPointer(), for the others return the only
core pointer.
https://bugzilla.gnome.org/show_bug.cgi?id=621685
