https://drafts.fxtf.org/geometry/#ref-for-dom-dompointreadonly-w-1

[[
    DOMPoint matrixTransform(optional DOMMatrixInit matrix);
]]

Why not return DOMPoint or DOMPointReadOnly accordingly?

For example, when implemented in JavaScript, then instead of

```
matrixTransform(matrix) {
  let result = new DOMPoint(this)
}
```

we would write

```
matrixTransform(matrix) {
  let result = new this.constructor(this)
}
```

so that it picks the right constructor.