Draft: Catch up with wlroots 0.18.x

Let's track wlroots git so we don't build up that much legacy again:

• Currently based on !524 (merged)
• Currently based on !535 (merged)
• Need to fix thumbnail renderer (https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4211)
• give the xdg-shell changes a 2nd look as we can simplify some things (see e.g. sway commit https://github.com/swaywm/sway/pull/7777)

We can pull some of this into 0.17.x based releases even.

Reference: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3760