Auto merge of #21750 - Manishearth:biquad, r=ferjm

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

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/21750)
<!-- Reviewable:end -->

components/script/dom/gainnode.rs

@@ -48,7 +48,7 @@ impl GainNode {
             node.node_id(),
             ParamType::Gain,
             AutomationRate::A_rate,
-            1.,       // default value
+            *options.gain,  // default value
             f32::MIN, // min value
             f32::MAX, // max value
         );