[Feature Request] Support for rotated monitors/output transforms

[ramosdetrigo]

I use one of my monitors as a vertical display rotated by 90°. I can rotate the output in niri, kde, etc, but not in noctalia-greeter, so the output on that monitor ends up sideways on the login screen. It would be nice to also have support for it in the greeter.

niri config as reference:

output "PNP(TXD) HDMI 0x00005445" {
    position x=0 y=0
    focus-at-startup
    mode "1440x900@74.984"
}

output "LG Electronics D2342P SerialNumber" {
    transform "90" // this rotates the secondary monitor's output by 90º
    position x=1440 y=-900
}