compositor: Remove meta_compositor_window_[un]mapped

We no longer unmap the toplevel windows during normal operation. The
toplevel state is tied to the window's lifetime.

Call meta_compositor_add_window / meta_compositor_remove_window instead...
window-actor: Remove suspicious optimization

Calling get_paint_volume() should not modify internal state.
surface-actor: Move unobscured_region processing here
window-actor: Never unredirect when under Wayland
window-actor: Simplify the unredirected check in cull_out
compositor: Simplify the unredirected window management code
window-actor: Flip set_redirected around

I know it's confusing with the triple negative, but unredirected is how
we track it elsewhere: we have an 'unredirected' flag, and 'should_unredirect'.
window-actor: Remove old unused APIs
Move position-changed / size-changed signals to the MetaWindow

They fit more appropriately over here...
surface content
xwayland: Update the surface actor for the window on set_window_id

We need to do this for XWayland windows, since we only get the event
telling us they're an XWayland window after the compositor knows about
the window.
window-actor: Don't use TFP under XWayland

Simply have a NULL surface actor until the set_window_id arrives...