Add spline support to `jxl_from_tree`
Add spline support to `jxl_from_tree`

Example image (produces the same image as
`third_party/testdata/jxl/splines.png`):

    Width 320
    Height 320
    XYB

    Spline
        0.03125 0.00625 0.003125 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
        1 0.321875 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
        1 0.24375 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
        0.3125 0 0 0.0625 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
        9 54
        118 159
        97 3
        10 40
        150 25
        120 300
    EndSpline

    - Set 0

libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl