Avalonia: fix HiDPI/Retina scaling for embedded window on macOS.
This MR fixes an Avalonia regression first noticed in 11.2.5 version.
For some reason, MainWindow.OnScalingChanged no longer sets Program.DesktopScaleFactor to 2.00 on HiDPI/Retina displays, which results in a pixelated look and defeats the purpose of bilinear scaling.
If I had to guess, Avalonia probably changed the behavior of OnScalingChanged, which is not called on initialization. Either way, this fix works.