256 segments max
8 vertices per segment (3 bits), index diff between adjacent segments must be -16 .. 16 (excluding 0, 5 bits)
segments must be convex
adjacent segments are either +-8 or they have a special bit set indicating that we need a 8 bit segment index

7 6 5 4 3 2 1 0
          -----
          | diff - 1 (1..8) if flag == 0
        -
        | sign flag
      -
      | flag (0 if diff from -8 to 8, 1 if adjacent segment byte following)
-----
  | vertex index
