fix CanvasRenderer#getWindowSize being able to return {undefined, undefined} thus causing errors in math down the line

static/canvas-renderer.js

@@ -54,6 +54,7 @@ class CanvasRenderer extends HTMLElement {
     }
 
     getWindowSize() {
+        if (this.width == null || this.height == null) return { width: 0, height: 0 };
         return {
             width: this.width,
             height: this.height,