Since we do not ever minimize or otherwise unmap clients (except when
destroying them), we don't need to track mapped/unmapped state. The
destroy case is captured in checking whether the surface has a buffer
attached to it, before requesting its texture.
This makes it easier to extend in the future. Also, it will make it
easier to add (un)install targets and eventually, distribute files such
as systemd services.
It appears that wlr_xdg_toplevel_set_maximized does not actually change
the size of the surface. Rather, it only makes the surface think it is
maximized. Hence, we need to set the size to that of the output
manually.
