Obtains the surface underneath this, returning the location of the device in
win_y in double precision.
Returns null if the surface tree under this is not known to GDK (for example, belongs to another application).
pointer Device to query info to.
return location for the X coordinate of the device location, relative to the surface origin, or null.
return location for the Y coordinate of the device location, relative to the surface origin, or null.
the Surface under the device position, or null.