The following characteristics of Bobs can be changed dynamically between
calls to DrawGList():

  * To change the location of the Bob in the RastPort drawing area,
    adjust the X and Y values in the VSprite structure associated with
    this Bob.

  * To change a Bob's appearance, the pointer to the ImageData in the
    associated VSprite structure may be changed.  Note that a change in
    the ImageData also requires a change or recalculation of the
    ImageShadow, using InitMasks().

  * To change a Bob's colors modify the PlanePick, PlaneOnOff or Depth
    parameters in the VSprite structure associated with this Bob.

  * To change a Bob's display priorities, alter the Before and After
    pointers in the Bob structure.

  * To change the Bob into a paintbrush, specify the SAVEBOB flag in the
    Bob.Flags field.

    Changes Are Not Immediately Seen.
    ---------------------------------
    Neither these nor other changes are evident until SortGList() and
    then DrawGList() are called.