Reintroduce 0.14.2 behavior for splines inner elements

Bevy 0.14.2 -> 0.15

In this version,

`let mut catmull = CubicCardinalSpline::new_catmull_rom(points.clone()).to_curve();`

the CubicCardinalSpline won't let me access the undelrying segments, which is needed here because I modified the poitns after the initial calculation. it was possible in 0.14.2 for, e.g:

`catmull.segments.iter_mut().map(..){..}`

Now, we only have the option to iterate over segments or push a new segment but not to modify existing points.

May you reenable this behavior please ?

`catmull.segments.0` or `catmull.segments.get_mut()` ?

Thanks