[FrameBuffer] Expose the DisplayScale property on the builder

[jeromelaban]

When I debug my app with WSL2, my app renders with small text.

I saw this information on Uno doc, but I don't know where to call FrameBufferHost.DisplayScale.

Originally posted by @ArchieCoder in #16736

[jeromelaban]

Thanks for the report. Indeed, this is not accessible from the SkiaBuilder, so for now you should be setting the environment variable instead.

Something like:

export UNO_DISPLAY_SCALE_OVERRIDE=2

or inline with bash using:

UNO_DISPLAY_SCALE_OVERRIDE=2 dotnet myapp.dll

For the debugging, you can use the launchSettings.json environment variables as well.