fix win32 square shadow border appearing on fluent menus.
fix win32 square shadow border appearing on fluent menus.

AvaloniaUI/Avalonia,Perspex/Perspex,wieslawsoltes/Perspex,SuperJMN/Avalonia,wieslawsoltes/Perspex,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,grokys/Perspex,SuperJMN/Avalonia,SuperJMN/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Perspex,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,Perspex/Perspex,wieslawsoltes/Perspex,SuperJMN/Avalonia,grokys/Perspex,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex,wieslawsoltes/Perspex,SuperJMN/Avalonia,jkoritzinsky/Avalonia