only add audioworklet modules once (#381)

This Promise `return ac.audioWorklet.addModule(objectURL)` was throwing an error if the worklet with that name had already been registered on the page. We run into this issue when running p5 in "instance mode" (i.e. on the documentation examples at p5js.org).

Author: therewasaguy

src/audioWorklet/index.js

@@ -6,6 +6,8 @@ const moduleSources = [
 ];
 const ac = p5sound.audiocontext;
 
+let initializedAudioWorklets = false;
+
 function loadAudioWorkletModules() {
   return Promise.all(moduleSources.map(function(moduleSrc) {
     const blob = new Blob([moduleSrc], { type: 'application/javascript' });
@@ -15,13 +17,16 @@ function loadAudioWorkletModules() {
 }
 
 p5.prototype.registerMethod('init', function() {
+  if (initializedAudioWorklets) return;
   // ensure that a preload function exists so that p5 will wait for preloads to finish
   if (!this.preload && !window.preload) {
     this.preload = function() {};
   }
+
   // use p5's preload system to load necessary AudioWorklet modules before setup()
-  this._preloadCount++;
+  this._incrementPreload();
   const onWorkletModulesLoad = function() {
+    initializedAudioWorklets = true;
     this._decrementPreload();
   }.bind(this);
   loadAudioWorkletModules().then(onWorkletModulesLoad);