kittest: Add UPDATE_SNAPSHOTS=force (#7508)

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.

Author: emilk