Create seemingly necessary subcompositor

[gwimm]

Both alacritty and foot want them, tinywl does it, so kiwmi now does it.

[kuruczgy]

The subcompositor not being created is a breaking change that wlroots 0.16.0 introduced: https://gitlab.freedesktop.org/wlroots/wlroots/-/releases/0.16.0:

wlr_subcompositor is not created automatically anymore on calling wlr_compositor_create(); use wlr_subcompositor_create().

As such, this should not be a separate PR, but included together with #71