Return true if this result is a "singleton" result. A singleton result is a type of
result that is can only be reported once per execution of a job. The job verifies
that singleton results are supplied at most once per job execution. Most results
should return false. The returned value should be the same for every instance of a
type, it should not depend on the state of the instance.
Singleton results should only be used when the computation of the result produces
all of the result data. As an example, a 3D engine might assign entities to lights,
and each unique configuration of lights on an entity is a "light group". It makes
more sense to provide a single result that describes all light groups than
individual results for each group. They are packed into a single result because
each group is dependent on the others to guarantee its uniqueness.
As a counter example, computing potentially visible sets for a 3D engine should not
be a singleton result. A result that contains the view and set of visible entities
is a self-contained result, other views or cameras do not affect the PVS results.
The default implementation returns false. Subtypes are free to override it, but it
must return the same value for all instances of a given type.