This was initially a PR to add kitdiff, but this now lives in it's own
crate: https://github.com/rerun-io/kitdiff
I needed to make the image loaders public, this way it's possible to
compose image loaders together (which allowed me to create a image diff
loader that uses two other image loaders). But you can't use the
`ctx.try_load_image` since that would deadlock, so you have to store a
reference to the other loader in the wrapping loader.
9150b9342d