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.