When we add support for weak materials it's expected that Clutter will
want to attach them as private data to other materials and it needs a
mechanism to determine when a weak material should be re-created because
its parent has changed somehow.
This adds the concept of a material age (internal only currently) which
increments whenever a material is modified. Clutter can then save the
age of the material which its weak materials are derived from and later
determine when the weak material may be invalid.
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