Add warning to wayland buffer

...to prevent devs from using it as a `ws_buffer` and filing bug-reports
because the method segfaults.
Fix wayland buffer usage in surface commit code
Merge pull request #634 from neithernut/add-compositor-wayland-buffer-warning

Add warning to wayland buffer
Add missing NULL-check
Fix handling of completely new dag tab trees
Rewrite add_roots_for()
Make depth a signed value
Remove old unused static function
Rewrite dag node removal