Applied [ 654253 ] wxGTK: fix to wxGetMousePosition()

wxGetMousePosition() returns -999, -999 when the mouse
pointer is not over a GTK window. This is apparently
because it uses the current window as a handle to get
to the X display. This is not necessary, however;
GDK_DISPLAY() can be used. With
this patch wxGetMousePosition() works regardless of
where on the screen the mouse is.

Jay Berkenbilt (jay_berkenbilt) modified by JACS

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@18250 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775