feat: option to pass in the number of input channels to Panner

fixes #609

Tone/component/channel/Panner.test.ts

@@ -67,5 +67,20 @@ describe("Panner", () => {
 				expect(r[0]).to.be.closeTo(0.707, 0.01);
 			});
 		});
+
+		it("can chain two panners when channelCount is 2", () => {
+			return Offline(() => {
+				const panner1 = new Panner({
+					channelCount: 2,
+				}).toDestination();
+				const panner0 = new Panner(-1).connect(panner1);
+				new Signal(1).connect(panner0);
+			}, 0.1, 2).then((buffer) => {
+				const l = buffer.toArray()[0];
+				const r = buffer.toArray()[1];
+				expect(l[0]).to.be.closeTo(1, 0.01);
+				expect(r[0]).to.be.closeTo(0, 0.01);
+			});
+		});
 	});
 });

Tone/component/channel/Panner.ts

@@ -6,6 +6,7 @@ import { readOnly } from "../../core/util/Interface";
 
 interface TonePannerOptions extends ToneAudioNodeOptions {
 	pan: AudioRange;
+	channelCount: number;
 }
 
 /**
@@ -55,7 +56,7 @@ export class Panner extends ToneAudioNode<TonePannerOptions> {
 		// this is necessary for standardized-audio-context
 		// doesn't make any difference for the native AudioContext
 		// https://github.com/chrisguttandin/standardized-audio-context/issues/647
-		this._panner.channelCount = 1;
+		this._panner.channelCount = options.channelCount;
 		this._panner.channelCountMode = "explicit";
 
 		// initial value
@@ -65,6 +66,7 @@ export class Panner extends ToneAudioNode<TonePannerOptions> {
 	static getDefaults(): TonePannerOptions {
 		return Object.assign(ToneAudioNode.getDefaults(), {
 			pan: 0,
+			channelCount: 1,
 		});
 	}