Using a new GetWindowStyleMasks driver entry to get the style masks to be used to mask out non-client decorations which are hidden by the host decorations.
This is done after WindowPosChanging, as winex11 needs to update its managed flag before computing the visible rect, this means that the visible rect might change between WindowPosChanging and WindowPosChanged. It should hopefully be fine.
-- v3: win32u: Move the "Decorated" driver registry option out of the drivers. win32u: Move visible rect computation out of the drivers. winemac: Return a macdrv_window_features from get_cocoa_window_features. winex11: Use the driver rects to convert from host visible to window rect. winex11: Keep a window_rects struct in the driver window data. win32u: Keep SetIMECompositionWindowPos with other IME entries.