Use git lfs for reference images

## Feature Description

Consider using Git Large File Storage (https://git-lfs.com/) for our reference images for VRT tests. This will let us reduce the size of our repository which is 2.41 GiB:

```
$ git count-objects -vH
```
```
count: 590
size: 3.15 MiB
in-pack: 432771
packs: 13
size-pack: 2.41 GiB
prune-packable: 137
garbage: 0
size-garbage: 0 bytes

```

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## Acceptance criteria

*

## Implementation Brief

* [ ]

### Test Coverage

*

## QA Brief

*

## Changelog entry

*

The issue identifies VRT reference images and the `git count-objects -vH` output as the starting points. Investigate how Git LFS would store and retrieve those images, then verify that visual regression tests continue to work while the repository size is reduced.

Written by the indexing model from the issue text.