Cellular distance noise computation is independent from cell distances

The function `single_cellular` is used to compute 'cellular' noise for `CellularReturnType::CellValue` and `CellularReturnType::Distance`. However, most of the code in the function is essentially dead - the return value is currently only determined by the input coordinates and the RNG seed (code copied from https://github.com/amethyst/bracket-lib/blob/master/bracket-noise/src/fastnoise.rs#L3857-L3862):
```rust
match self.cellular_return_type {
CellularReturnType::CellValue => {
self.val_coord_2d(self.seed as i32, x as i32, y as i32)
}
_ => 0.0,
}
```

Intuitively, I expect that when writing `distance` to a new 'minimal' value, also the corresponding coordinates `xi` and `yi` should be remembered and passed to `val_coord_2d`. Something entirely different from `return 0.0` is likely necessary for other values of `CellularReturnType`. Looking at the original source in https://github.com/Auburn/FastNoiseLite/blob/master/C/FastNoiseLite.h#L1402 seems to confirm this.