Add RDP scaling factor as implemented by FreeRDP

Copied from #654 (closed) comment posted by @rubin110

Is there a way to have Remmina specify a scaling factor back to the host machine (Windows)?

I recently got a new laptop with an HDPI display. When I try to connect to my host machine (which runs some photo editing software I can't get for Linux), I get a very large native 1:1 screen making most of the UI in Windows too tiny to use. I can specify a smaller resolution in Remmina and scale on the client side, however this is problematic as it causes unneeded stretching and makes editing photos accurately impossible. If I jump into the display settings (by right clicking on the desktop), the scaling options on these options are disabled due to using RDP.

I see that freerdp offers a scaling-desktop option. Is there anyway to pass this option from the Remmina interface?

Thanks!