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/src/load.rs

@@ -387,7 +387,7 @@ pub type ImageLoadResult = Result<ImagePoll>;
 /// An `ImageLoader` decodes raw bytes into a [`ColorImage`].
 ///
 /// Implementations are expected to cache at least each `URI`.
-pub trait ImageLoader {
+pub trait ImageLoader: std::any::Any {
     /// Unique ID of this loader.
     ///
     /// To reduce the chance of collisions, include `module_path!()` as part of this ID.
@@ -517,6 +517,16 @@ impl TexturePoll {
             Self::Ready { texture } => Some(texture.id),
         }
     }
+
+    #[inline]
+    pub fn is_pending(&self) -> bool {
+        matches!(self, Self::Pending { .. })
+    }
+
+    #[inline]
+    pub fn is_ready(&self) -> bool {
+        matches!(self, Self::Ready { .. })
+    }
 }
 
 pub type TextureLoadResult = Result<TexturePoll>;