Fix LayoutUpdated subscriptions.
Fix LayoutUpdated subscriptions.

And don't call `OnEffectiveViewportChanged` if viewport unchanged.

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