We try to exempt CSD windows from being forced fullscreen if they are
undecorated and the size of the screen; however, we also catch almost
all windows that *do* need to be forced fullscreen in this check, since
they also have decorations turned off.

Identify actual CSD windows by checking whether _GTK_FRAME_EXTENTS is set -
GTK+ will always set this on CSD windows even if they have no invisible
borders or shadows at the current time.

We explicitly turn off the legacy-fullscreen check for native wayland windows
so we don't start legacy-fullscreening them if the new
meta_window_is_client_decorated() is later made more accurate.

https://bugzilla.gnome.org/show_bug.cgi?id=723029