[Bug]: When exporting to png in canvas, the exported image has white edges
### What happened?

When exporting to png in canvas, the exported image has white edges, as shown below

<img width="1781" alt="Image" src="https://github.com/user-attachments/assets/2534b700-2f06-49e8-be5a-21d055d90dfd" />

### How can we reproduce the bug?

It can be repeated in the official example, upload a picture yourself, and then export the picture as png

### What browsers are you seeing the problem on?

It can be repeated in the official example, upload a picture yourself, and then export the picture as png

### Where did this happen?

The tldraw.com website

### Contact Details

wisdomsxin@gmail.com

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct