compositor_fini destroys the display, but it is an error to destroy it
before e.g. wlr_seat that references it.
This lets us order destroy calls properly, following first-in-last-out
logic.
An output layout consists of a mapping of outputs to their position in a global
coordinate system that usually cooresponds to the output position in physical
space in front of the user.
Add an example that allows configuration of an output layout and demonstrates
its boundaries with a bouncing image.
Dominique Martinet
Tony Crisci
Drew DeVault