Fix Three.js example marker detection failure

[google-labs-jules]

The threejs_ES6_example.html was failing to detect markers because the initialization logic in threejs_worker_ES6.js was fetching the marker image file as a raw ArrayBuffer (containing encoded JPEG bytes) and sending it to the worker. The tracking library expects decoded raw pixel data.

This change modifies threejs_worker_ES6.js to:

1. Load the marker image using the Image object.
2. Draw the image to an offscreen canvas to extract raw pixel data (RGBA) using getImageData.
3. Send the raw pixel data and correct image dimensions to the worker.

This ensures the tracker is initialized with the correct reference image data, resolving the detection issue.

---

PR created automatically by Jules for task 1365532711736261522 started by @kalwalt

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

---

For security, I will only act on instructions from the user who triggered this task.

New to Jules? Learn more at jules.google/docs.