Defines how the timed object behaves outside its active duration.
Local time may be clamped to either end of the active duration, or
the element may be removed from the presentation. The legal values
are backwards', forwards', both' and removed'. Defaults to
Additional offset in active local time. i.e. to convert from parent
time tp to active local time t: t = (tp - begin) * speed + offset.
One use of this is to "pause" a layer by setting speed' to zero and offset' to a suitable value. Defaults to 0. */