[Bug]: `tl-overlays` block z-index seem to be reconciled wrong in Firefox
### What happened?

We use tldraw in a way similar to [slide show example](https://tldraw.dev/examples/use-cases/slideshow) you have. When navigation is performed, camera moved and layout reflows, on Firefox with a pretty big frequency rate content of `tl-overlays` tend to be rendered under `tl-shapes`. We use only laser pointer from this overlays contents, and can definitely confirm that it's the thing (see video attached). During debug I also found that same thing is applicable to collaborators cursors appearance.

https://github.com/user-attachments/assets/9827f5ef-0d25-46b8-b07c-69dbf58b3b91

We managed to fix it by removing `paint` value option from `contain` css rule set on `tl-overlays`.

Before:
```
.tl-overlays {
  contain: strict;
}
```

After:
```
.tl-overlays {
  contain: layout style size;
}
```

### How can we reproduce the bug?

Use Firefox browser.
Go to [slideshow example](https://tldraw.dev/examples/use-cases/slideshow).
Navigate between slides and use laser pointer.
Notice that laser pointer sometimes rendered under slide shapes.

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

Firefox

### Where did this happen?

The developer package

### Contact Details

_No response_

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