I moved away from the `Window` struct all the logic to handle task
sources, into a new struct called `TaskManager`. In a happy world, I'd
be able to just have there two functions, of the types:
```rust
fn task_source<T: TaskSource>(&self, name: TaskSourceName) -> Box<T>
fn task_source_with_canceller<T: TaskSource>(&self, name: TaskSourceName)
-> (Box<T>, TaskSourceCanceller)
```
And not so much duplicated code. However, because TaskSource can't be a
trait object (because it has generic type parameters), that's not
possible. Instead, I decided to reduce duplicated logic through macros.
For reasons[1], I have to pass both the name of the function with
canceller and the name of the function without, as I'm not able to
concatenate them in the macro itself. I could probably use
`concat_idents` to create both types already defined and reduce the
amount of arguments by one, but that macro is nightly only. At the same
time, not being able to declare macros inside `impl` forces me to pass
`self` as an argument.
All this makes this solution more verbose than it would be ideally. It
does reduce duplication, but it doesn't reduce the size of the file.
[1](https://github.com/rust-lang/rust/issues/29599)
Implement BiquadFilterNode
A bunch of tests still fail but some of it may be a timing issue, looking at it the tests are *at least* affected by https://github.com/servo/servo/issues/21659 (changing how they work to avoid problems from that does not make them pass but does change the exact value of the error), so I feel like I should fix that first before investigating these.
r? @ferjm
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