The introduction of the StageManager in 0.8 implied that the first Stage
instance to be created was automatically assigned the status of "default
stage". This was all well and good, since the default stage was created
behind the curtains by the initialization sequence.

Now that the initialization sequence does not create a default stage any
longer, it means that the first stage created using clutter_stage_new()
gets to be the default, and all special and warm and fuzzy - which also
means that the first stage created by clutter_stage_new() cannot be
destroyed or handled as any other stage. Whoopsie.

Let's go back to the old semantics: the stage created by the first
invocation of clutter_stage_get_default() is the default stage, and
nothing else can be set as default. One day we'll be able to break the
API and the whole default stage business will be a thing of the past.