WIP: Adjust to wlroots 0.15
Built on top of !280 (closed)
Vanilla wlroots fails to initialize EGL and fallbacks to pixman (where everything is unusably slow and thumbnails don't work): see https://github.com/swaywm/wlroots/issues/3275. In this MR, submodule points to a tree with some commits reverted, making EGL work again.