Merge wlroots backend into master

The all other backends replacing wlroots backend on the separate feature branch with the same name now largely works as expected. There are only minor issues which we can continue to work on on the master branch.

This finalizes a challenging move to wlroots as hardware/platform backend instead of having individual backends for each platform on our own. While the cost for this move was not insignificant, long-term it is assumed that that the advantage of sharing code and solutions with other wlroots based compositors will offset that manifold.