fix: default background is rendered all the time

ts/src/renderer.ts

@@ -96,6 +96,9 @@ export class MonoDisplayRenderer {
     let cleared = false;
 
     let elementId = 0;
+    // default off
+    this.ctx.fillStyle = this.opts.offColor;
+    this.ctx.fillRect(0, 0, this.ctx.canvas.width, this.ctx.canvas.height);
 
     for (const section of file.sections) {
       if (section.sectionType === SectionType.ElementsTimespan) {