layer-shell: Move handling of mapped property out of commit handler

We need to be able to tell whether the surface is mapped or not before handle_surface_commit is called, since unmapping causes cursor focus to be updated, which in turn could end up choosing the surface that's being unmapped as the focused one again. We can't rely on wlr_layer_shell's mapped property, because it only gets updated already after unmap handler is called. Therefore, handle mapped status directly in map/unmap handlers.

With that in place, we can ignore unmapped surfaces in layer_surface_at, which fixes #277 (closed).