winemac: Post WINDOW_FRAME_CHANGED with the non-fullscreen frame when exiting of fullscreen mode begins.

We had been posting it when exiting fullscreen mode ended.  However, certain
events during exiting could provoke the back-end to assert the window frame as
it knows it, which would be the one from full-screen mode.  This would be
handled by the Cocoa thread after exiting full-screen mode.  So, the window
would animate to its pre-fullscreen frame and then spontaneously go back to
covering the screen.  This would be Windows-style fullscreen rather than
Cocoa-style and there'd be no obvious way to get out.

The problem occurs on macOS 10.12 (Sierra) due to a change in what methods it
calls on the window while exiting fullscreen.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>