gtk/win32: use gdk_win32_surface_get_handle

gtk/win32: use gdk_win32_surface_get_handle

As I propose to deprecate gdk_win32_surface_get_impl_hwnd() next,
replace it with the alternative.

The main difference between the two functions is that
gdk_win32_surface_get_impl_hwnd() fails gracefully by returning NULL if
the surface is not of the win32 implementation.

All the surfaces should be native surfaces here, and the existing code
doesn't seem to deal with NULL case anyway.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>