Add ladder filter example

examples/Synthesis/LadderFilter/LadderFilter.ino

+// Ladder filter demo with continuous 3-saw drone into the filter
+// with separate LFOs modulating filter frequency and resonance.
+// By Richard van Hoesel
+// https://forum.pjrc.com/threads/60488?p=269756&viewfull=1#post269756
+
+#include <Audio.h>
+
+AudioSynthWaveform       waveform1;
+AudioSynthWaveform       waveform2;
+AudioSynthWaveform       waveform3;
+AudioMixer4              mixer1;
+AudioFilterLadder        filter1;
+AudioSynthWaveform       lfo1;
+AudioSynthWaveform       lfo2;
+AudioOutputI2S           i2s1;
+//AudioOutputUSB         usb1;
+AudioControlSGTL5000     sgtl5000_1;
+
+AudioConnection patchCord1(waveform1, 0, mixer1, 0);
+AudioConnection patchCord2(waveform2, 0, mixer1, 1);
+AudioConnection patchCord3(waveform3, 0, mixer1, 2);
+AudioConnection patchCord4(mixer1, 0, filter1, 0);
+AudioConnection patchCord5(lfo1, 0, filter1, 1);
+AudioConnection patchCord6(lfo2, 0, filter1, 2);
+AudioConnection patchCord7(filter1, 0, i2s1, 0);
+AudioConnection patchCord8(filter1, 0, i2s1, 1);
+//AudioConnection patchCord9(filter1, 0, usb1, 0);
+//AudioConnection patchCord10(filter1, 0, usb1, 1);
+
+void setup() {
+  AudioMemory(10);
+  sgtl5000_1.enable();
+  sgtl5000_1.volume(0.6);
+
+  filter1.resonance(0.55);
+  filter1.frequency(4000);
+  waveform1.frequency(50);
+  waveform2.frequency(100.1);
+  waveform3.frequency(150.3);
+  waveform1.amplitude(0.3);
+  waveform2.amplitude(0.3);
+  waveform3.amplitude(0.3);
+  waveform1.begin(WAVEFORM_BANDLIMIT_SAWTOOTH);
+  waveform2.begin(WAVEFORM_BANDLIMIT_SAWTOOTH);
+  waveform3.begin(WAVEFORM_BANDLIMIT_SAWTOOTH);
+  lfo1.frequency(0.2);
+  lfo1.amplitude(0.985);
+  lfo1.phase(270);
+  lfo2.frequency(0.07);
+  lfo2.amplitude(0.55);
+  lfo2.phase(270);
+}
+
+void loop() {
+  Serial.print("CPU Usage: ");
+  Serial.print(AudioProcessorUsageMax());
+  Serial.println("%");
+  AudioProcessorUsageMaxReset();
+  delay(1000);
+}