Use the dirty rect from BeginPaint on WM_PAINT.
Use the dirty rect from BeginPaint on WM_PAINT.

The one returned by `GetUpdateRect` at this point will be empty, from
the win32 docs:

> BeginPaint automatically validates the update region, so any call to
GetUpdateRect made immediately after the call to BeginPaint retrieves an
empty update region.

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