Gstreamer backend returns channel as single bit mask (ie 1, 2, 4, 8, 32 etc).
Progress callback was using this mask as plain channel index, thus storing decoded
audio in wrong channel.
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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