The PrevSeq, NextSeq, PrevComp and NextComp linkages have no bearing on
the order in which AnimComps in any given video frame are drawn.  To
specify the inter-component priorities (so that the closest objects appear
frontmost) the Before and After pointers in the initially active
AnimComp's underlying Bob structure are linked in to the rest of the
system, as described previously in the discussion of Bobs.

This setup needs to be done once, for the initially active AnimComps of
the AnimOb only.

The animation system adjusts the Before and After pointers of all the
underlying Bob structures to constantly maintain the inter-component
drawing sequence, even though different components are being made active
as sequencing occurs.

These pointers also assure that one complete object always has priority
over another object. The Bob Before and After pointers are used to link
together the last AnimComp's Bob of one AnimOb to the first AnimComp's Bob
of the next AnimOb.