allow pianoroll bg to be transparent

repl/src/pianoroll.mjs

@@ -15,6 +15,7 @@ Pattern.prototype.pianoroll = function ({
   this.draw(
     (ctx, events, t) => {
       ctx.fillStyle = background;
+      ctx.clearRect(0, 0, w, h);
       ctx.fillRect(0, 0, w, h);
       events.forEach((event) => {
         const isActive = event.whole.begin <= t && event.whole.end >= t;