gdksurface-win32.c: Fix window menu positioning (cherry-pick for gtk-4.0)

We ought to get the coordinates of where the window menu should be displayed using gdk_win32_surface_get_root_coords(), instead of rounding the position that we obtained with gdk_event_get_position().

Also rename items a bit in the same function, and call gdk_event_get_event_type() for consistency with the other backends.

Fixes issue #3704 (closed).

(cherry picked from commit e941e2be)