`nseed` function not working
The[ `Perlin::set_seed`](https://docs.rs/noise/0.7.0/noise/struct.Perlin.html#method.set_seed) function used in the `nseed` function returns a new `Perlin` instance which is not assigned to the `Math` instance, so the Perlin seed is not really changed.  https://github.com/kitao/pyxel/blob/779dc3ff29b01289cf3217610e6fdbd947cbc027/crates/pyxel-core/src/math.rs#L79  To fix it, it is possible to use an approach similar to the `rseed` function like:

```rust
fn nseed(seed: u32) {
    let instance = Math::instance();
    instance.perlin = instance.perlin.set_seed(seed);
}
```

Or using the latest `noise` version (0.8.2):

```rust
fn nseed(seed: u32) {
    Math::instance().perlin = Perlin::new(seed);
}
```