The root vsprite structure is set up as described earlier for true
VSprites, with the following exceptions:

             y, x  bob position is always in pixels that are the same
                   resolution as the display.

            flags  for bobs, the vsprite flag must be cleared.  saveback
                   or overlay can also be used.

    height, width  bob pixels are the size of the background pixels. the
                   Width of Bobs may be greater than one word.

            depth  the depth of a bob may be up to as deep as the
                   playfield, provided that enough image data is provided.

        imagedata  this is still a pointer to the image, but the data
                   there is organized differently.

        sprcolors  this pointer should be set to null for bobs.

            vsbob  this is a pointer to the bob structure set up as
                   described below.