Correctly generate child tile ranges for XYZ

Instead of incrementing and then doubling, calculate the child tile range by doubling and then incrementing.

With this change, tile coord [0, 0, 0] has the four following children:

    [1, 0, 0]    [1, 1, 0]

    [1, 0, 1]    [1, 1, 1]

Without this change, tile coord [0, 0, 0] had the nine following children:

    [1, 0, 0]    [1, 1, 0]    [1, 2, 0]

    [1, 0, 1]    [1, 1, 1]    [1, 2, 1]

    [1, 0, 2]    [1, 1, 2]    [1, 2, 2]