gdk/win32: Fix window display and resizing

gdk/win32: Fix window display and resizing

Have an implementation of ->request_layout() and ->compute_size() for the Win32
surface backend so that we can properly display and move and resize the
windows, as we request from the Win32 APIs.

Hxndling Aerosnap properly is mostly done except for snap_up(), which needs to
to be looked at later.