spectrumPackages.sys-vms.comp: switch to tinywl

Recent versions of wlroots (not yet in Spectrum's Nixpkgs) make it
impossible to use Wayfire as a Wayland client over Virtio Wayland,
because of an architectural change in wlroots that means dma-bufs are
now required to use the GLES2 renderer, which is the only one Wayfire
supports, with the Wayland backend.  Sommelier as a compositor does
not (and cannot) support the zwp_linux_dmabuf_v1 Wayland protocol this
requires, because to do so it would have to be able to send dma-bufs
allocated by the guest client over Virtio Wayland.  (This isn't
possible because of general difficulties with guest->host memory
sharing.)

The "comp" VM as it stands is designed to be used with spectrum-vm, as
a demo that people can run on their non-Spectrum systems, and see a
Wayland compositor appear as a Wayland window in their host
compositor.  Since it's no longer possible to use Wayfire for this use
case, let's use tinywl here instead, since we have no requirements
that it doesn't fulfill as a minimum-viable compositor.

This doesn't change anything regarding non-demo uses of Spectrum.
Running a Wayland compositor itself as a Wayland client is not an
activity that really makes sense in an actual Spectrum system.
Realistic use cases for Wayland compositors in Spectrum (on a host
system, or in a VM with a GPU passed through, or possibly in a VM with
virtio-gpu) are not affected by this limitation, since in all cases
the compositor would render directly to a GPU, not to a Wayland window
to be displayed by another compositor.

Message-Id: <20210701155446.1306212-3-hi@alyssa.is>

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