Remove 'map' handler for VirtViewerDisplay
    
    In order to solve several problems with sizing and resizing displays, a
    'map' handler was added to VirtViewerDisplay. The first time the map
    handler runs, its queues a resize to attempt to ensure that the window
    gets created at its desired size. Subsequent map events generate a call
    to _make_resizable(), which was an attempt to ensure that the window was
    always 'shrinkable' on the Microsoft Windows platform. Recent testing
    suggests that this _make_resizable() is not actually necessary on
    Windows anymore, since it is possible to shrink the display even when
    this call is removed.
    
    In addition, the call to _queue_resize() is a bit of an indirect
    solution to the problem of ensuring the proper size at startup. What we
    really want is to guarantee that the very first size request negotiation
    returns the desired size rather than the minimum size. In order to do
    this, we've added a flag to determine whether we've ever received a size
    request, and if not, we return our desired size, even if 'dirty' is not
    set.