#53 Unconditional initialization of SourceClip _startTime.

[Jim Trainor]

The SourceClip _startTime property is optional.

It should only be set if the source reference is to a
TimelineMobSlot or an EventMobSlot.

It should *not* be set if the reference is to a StaticMob
slot.

ImplAAFSourceClip::Initialize() sets _startTime
unconditionally, hence, it is wrong if the reference is to
a StaticMob slot.