When a window is shown, it may not have drawn its content into the backing
surface, yet.  Cocoa will draw the window, starting with its standard light
gray background and then the content view.  However, the content view won't
have anything to draw, yet, though, so the window background is not drawn over.

A short while later, usually, the app will paint its content into the window
backing surface and Cocoa will be told to redraw the window.  This works, but
the user can often see the flash of the window background color first.  This
is especially visible for windows with dark content.

Part of the fix is to set the window background to transparent until the
content view has actually drawn once since the window was shown.

That's not sufficient on its own, though.  We had disabled Cocoa's automatic
display mechanism for windows and put display on a display-link timer.  This
meant that the window was not actually cleared to its transparent color.  When
the window was shown, the Window Server displayed a white backing buffer.  It
is the app process which should fill that backing buffer with clear color but,
because we had disabled auto-display, that wasn't getting done at the same
time the window was displayed.  It was happening some time after.  Again, the
result was a visible flicker of white.

So, we now temporarily re-enable auto-display just before showing a window.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>