Use a structured clone holder to store rooted clones
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Implement document load cancellation
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Implement update source set & select image source
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It currently works by constructing from null (which will throw a runtime
error if there are non-defaultable members).
This changes it so that we no longer need a JSContext to construct this,
so it can be safely constructed. In the case of non-defaultable members,
this method simply does not exist.
Store vertex attribs data in DOM and optimise GetVertexAttrib
This is not an extremely useful change on its own but those things need to be stored on the DOM side to implement some draw checks anyway.
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DOM structs embed their parent type as their first field. This
introduces a `.parent()` method to the DOM struct that returns its first
field, and codegens a type assert that ensures that `.parent()` returns
the parent struct.
This generates:
On `#[dom_struct]`:
```rust
impl HasParent for Type {
type Parent = ParentType;
fn as_parent(&self) -> ParentType {
&self.first_field
}
}
```
In the codegen files:
```rust
impl Type {
fn __assert_parent_type(&self) {
let _: &ParentType = self.as_parent();
}
}
````
