Fix ClientToScreen on high-DPI.
Fix ClientToScreen on high-DPI.

Closes #445 (again).

punker76/Perspex,MrDaedra/Avalonia,jkoritzinsky/Avalonia,Perspex/Perspex,AvaloniaUI/Avalonia,jkoritzinsky/Perspex,AvaloniaUI/Avalonia,susloparovdenis/Avalonia,akrisiun/Perspex,jazzay/Perspex,susloparovdenis/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,OronDF343/Avalonia,susloparovdenis/Perspex,SuperJMN/Avalonia,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,wieslawsoltes/Perspex,SuperJMN/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,grokys/Perspex,Perspex/Perspex,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,wieslawsoltes/Perspex,susloparovdenis/Perspex,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,MrDaedra/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,wieslawsoltes/Perspex,SuperJMN/Avalonia,wieslawsoltes/Perspex,grokys/Perspex,OronDF343/Avalonia,SuperJMN/Avalonia,wieslawsoltes/Perspex