This only really affects OpenGL child windows.  GDI rendering to the window
surface is still only blitted to the window's content view.  The descendant
views don't draw and so are transparent, letting the content view show through.

Using Cocoa views for child windows fixes a problem where changes to the
position and visibility of child GL windows didn't properly affect the Cocoa GL
view.  Hiding, showing, and moving the top-level window affected the Cocoa
window and thus, indirectly, the GL view.  Moving the child GL window itself
was propagated to the GL view, so that worked.  But hiding, showing, or moving
any of the intervening ancestors of the child GL window didn't properly affect
the GL view.  Neither did hiding or showing the child GL window itself.

This also slightly improves the clipping of the GL view by its ancestors,
although it still doesn't work quite right due to Cocoa bugs.  There are also
remaining bugs with z-order among multiple GL views and clipping by overlapping
siblings.  I hope to eventually fix those using Core Animation layers, for
which this is a prerequisite.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>