Use correct size for root window with multiple monitors

Use correct size for root window with multiple monitors

The root window width and height have already been correctly
initialised in _gdk_root_window_size_init() to cover all monitors, so
don't incorrectly re-initialise using GetSystemMetrics(SM_C[XY]SCREEN)
which only gives the size of the primary monitor anyway. (See MSDN.)

This fixes at least gdk_screen_get_{width,height}() which indirectly
affects at least the positioning of combo box pop-up menus on multiple
monitors.