gdksurface-win32.c: Acquire root coordinates on drag/move

Hi,

A recent change in the begin_move() and begin_drag() APIs required an update to the GDK-Win32 backend, so that the move (and thus dragging/resizing) of Windows can be fixed, in particular on HiDPI displays. Basically we need to do an extra step to acquire the root coordinates based on the x and y surface coordinates that we pass in.

With blessings, thank you!