This patch brings the behavior of this method into line with the comment that
describes it and other methods in this class that have a similar test.
I discovered this bug while exercising this code from animation code and was
getting test failures due to returning -Infinity instead of the expected
Infinity.
This patch adds another implementation of BaseTimeDuration's ValueCalculator
template parameter that is careful to preserve Forever/-Forever values when
performing arithmetic.
It also defines a typedef for a specialization of BaseTimeDuration that uses
this new ValueCalculator definition.
Brian Birtles
Andrew McCreight