WIP - AudioOutputMixer to combine multiple streams

This runs but amplification is not working yet.  See MixerSample
for a very crufty example.

examples/MixerSample/MixerSample.ino

@@ -0,0 +1,64 @@
+#include <Arduino.h>
+#ifdef ESP32
+    #include <WiFi.h>
+#else
+    #include <ESP8266WiFi.h>
+#endif
+
+#include "AudioFileSourcePROGMEM.h"
+#include "AudioGeneratorWAV.h"
+#include "AudioOutputI2S.h"
+#include "AudioOutputMixer.h"
+
+// VIOLA sample taken from https://ccrma.stanford.edu/~jos/pasp/Sound_Examples.html
+#include "viola.h"
+
+AudioGeneratorWAV *wav[2];
+AudioFileSourcePROGMEM *file[2];
+AudioOutputI2S *out;
+AudioOutputMixer *mixer;
+AudioOutputMixerStub *stub[2];
+
+void setup()
+{
+  WiFi.mode(WIFI_OFF); 
+  Serial.begin(115200);
+  delay(1000);
+  Serial.printf("WAV start\n");
+  file[0] = new AudioFileSourcePROGMEM( viola, sizeof(viola) );
+  out = new AudioOutputI2S();
+  mixer = new AudioOutputMixer(32, out);
+  stub[0] = mixer->NewInput();
+  wav[0] = new AudioGeneratorWAV();
+  wav[0]->begin(file[0], stub[0]);
+  // Begin wav[1] later in loop()
+  Serial.printf("starting 1\n");
+}
+
+void loop()
+{
+  static uint32_t start = 0;
+  static bool go = false;
+
+  if (!start) start = millis();
+
+  if (wav[0]->isRunning()) {
+    if (!wav[0]->loop()) { wav[0]->stop(); stub[0]->stop(); Serial.printf("stopping 1\n"); }
+  }
+
+  if (millis()-start > 3000) {
+    if (!go) {
+      Serial.printf("starting 2\n");
+      stub[1] = mixer->NewInput();
+      wav[1] = new AudioGeneratorWAV();
+      file[1] = new AudioFileSourcePROGMEM( viola, sizeof(viola) );
+      wav[1]->begin(file[1], stub[1]);
+      go = true;
+    }
+    if (wav[1]->isRunning()) {
+      if (!wav[1]->loop()) { wav[1]->stop(); stub[1]->stop(); Serial.printf("stopping 2\n");}
+    }
+  }
+
+}
+