When we start the engine we haven't yet gotten any X11 motion events so
we don't yet know where our mouse cursor is located. Instead we now
query the X server for this information when we start and update the
appropriate values.
In addition when we move the window we also update the mouse position
based off of X server knowledge as we will also not have received any
mouse motion events.
this fixes#8145 (for X11 only)