don't set view->w and view->h when mapping xdg views

don't set view->w and view->h when mapping xdg views

these should only be set on commit. doing so before then confuses
code that expects these fields to be in sync with the scene tree,
such as `handle_commit`.

these were only being set so that `view_center` could read them to
compute a centered position, so instead we can simply forward the
values directly to `view_compute_centered_position` and `view_move`.