Audio: clean up event listeners and buffer view on destroy (#75)

The Audio constructor binds three window-level event listeners
(keydown / click / touchend) to drive startPlayback. They're only
removed when startPlayback() fires; if the Emulator is destroyed
before the user produces a gesture — or if an embedder recreates
the Emulator multiple times before that happens — the listeners
stayed attached to a stale Audio instance forever.

Audio also holds a TypedArray view (this.buffer) into the wasm
heap region returned by _get_audio_buffer_ptr. After Emulator
destroy() calls _emulator_delete + _free, that backing memory is
gone, and any held reference to the buffer view points at freed
memory.

Add Audio.destroy(): removes the gesture listeners and nulls out
the buffer view. Call it from Emulator.destroy() before
_emulator_delete so the cleanup happens while the wasm pointer is
still valid.

Author: kwigbo

docs/simple.js

@@ -194,6 +194,7 @@ class Emulator {
     this.cancelAnimationFrame();
     clearInterval(this.rewindIntervalId);
     this.rewind.destroy();
+    this.audio.destroy();
     this.module._emulator_delete(this.e);
     this.module._free(this.romDataPtr);
   }
@@ -658,6 +659,17 @@ class Audio {
     if (!this.started) { return; }
     Audio.ctx.resume();
   }
+
+  destroy() {
+    if (this.boundStartPlayback) {
+      window.removeEventListener('keydown',  this.boundStartPlayback, true);
+      window.removeEventListener('click',    this.boundStartPlayback, true);
+      window.removeEventListener('touchend', this.boundStartPlayback, true);
+      this.boundStartPlayback = null;
+    }
+    this.buffer = null;
+    this.started = false;
+  }
 }
 
 Audio.ctx = new AudioContext;