winemac: Check the window's display link after adding it as a child of another window, which may order it on screen.

This fixes a problem where child windows ("owned" windows in Windows
parlance) would never display their contents on OS X 10.8 or earlier.
Beginning with 10.9, Cocoa calls -windowDidChangeOcclusionState: when
the window becomes visible, which is why they display on that version
and later.

Reported by Huw Davies.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>