Override subpixel geometry

[viluon]

Much can be configured for outputs but subpixel geometry cannot be. It's not even reported in niri msg outputs, even though Smithay tries to detect it. On some monitors, detection can fail (Subpixel::Unknown) or can choose an incorrect value. Sway, for instance, lets the config override it.

Let's add an output configuration option for subpixel geometry to Niri as well.

[YaLTeR]

I'm not sure anything actually uses it