stack: query preferred size for last_visible_child

stack: query preferred size for last_visible_child

When animating, we might be changing the size allocation of the previous
stack child. However, we were not querying the size in the process
meaning you would often see warnings about allocating the size without
knowing what it should be.

This simply adds an innocuous size request, since responding to last_child
sizing requests is not all that critical in the transition.

https://bugzilla.gnome.org/show_bug.cgi?id=763900