param-source: fix cleanup of old events

lib/param-source.js

@@ -129,10 +129,11 @@ function ReducedParams (params, reducer) {
   this.params = params
   this.reducer = reducer
   this.events = []
+  this.context = getContext(params)
 }
 
 ReducedParams.prototype.onEvent = function (listener) {
-  var context = this.params[0] && this.params[0].context
+  var context = this.context
   var releases = this.params.map(param => ParamSource.isParam(param) && param.onEvent((event) => {
     var duration = 0
     var sampleTime = event.time
@@ -196,3 +197,11 @@ function cancelScheduledValues (source, time) {
 function last (array) {
   return array[array.length - 1]
 }
+
+function getContext (params) {
+  for (var i = 0; i < params.length; i++) {
+    if (params[i] && params[i].context) {
+      return params[i].context
+    }
+  }
+}