Support running on multiple outputs at once

[sin-ack]

This requires launching separate threads for each display because they will run at different frequencies. I looked up how to do this but forgot about it.

[sin-ack]

Looks like we need one event queue per output: https://wayland.freedesktop.org/docs/html/apb.html#Client-classwl__display

[bigFin]

This works for me on niri
#29