The shell module only needs to deal with attach when it's either the initial
attach or when the attach changes the size of the surface. In case of
initial attach, the shell needs to pick a position for the surface and a
place in the surface stack. We split this case out as a new shell->map
callback. The other case is split into the shell->configure callback,
where the shell may adjust the surface position or reject the new size.
Kristian Høgsberg
32e24cc628