kittest: Add UPDATE_SNAPSHOTS=force

[emilk]

This adds a new mode, UPDATE_SNAPSHOTS=force, which will lower the threshold to zero, overwriting every image that is not exactly the same.

Most comparisons has a threshold because different GPUs render slightly differently. However, setting that threshold accurately can be hard.

Sometimes a test will pass locally, but fail on CI. In those cases you want to force an update of the failing test. You can use UPDATE_SNAPSHOTS=force for that.

And sometimes a small change should update all images, but the change is so tiny that it falls under the threshold. Still, you want to make a point of showing that these images have changes. You can use UPDATE_SNAPSHOTS=force for that.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7508-emilkadd-updat-snapshots-force
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

Nice!