Make individual egui_extras image loaders public (#7551)

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.

crates/egui_extras/src/lib.rs

@@ -17,7 +17,7 @@ pub mod syntax_highlighting;
 #[doc(hidden)]
 pub mod image;
 mod layout;
-mod loaders;
+pub mod loaders;
 mod sizing;
 mod strip;
 mod table;