Add a wlr_output_set_subpixel()
Created by: ggreer
drmModeConnector.subpixel doesn't seem to detect subpixel order on many displays (especially laptops). Allow subpixel order to be manually set.
My plan is to make a corresponding PR for sway that adds a subpixel output option. Then https://github.com/swaywm/sway/issues/3163 will be fixed.
I'm very unfamiliar with the wlroots codebase so it's quite likely I made a mistake in this PR.