renderer: Change 'set_legacy_view()' to 'add_view()'

"Legacy" is a misleading name, it's just how the native backend and the
X11 backend behaves differently. Instead rename it to 'add_view()' and
add the sanity check to the caller.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1251
(cherry picked from commit 73a43636)