Add ImageLoader::has_pending and wait_for_pending_images (#7030)

With kittest it was difficult to wait for images to be loaded before
taking a snapshot test.
This PR adds `Harness::with_wait_for_pending_images` (true by default)
which will cause `Harness::run` to sleep until all images are loaded (or
`HarnessBuilder::with_max_steps` is exceeded).

It also adds a new ImageLoader::has_pending and
BytesLoader::has_pending, which should be implemented if things are
loaded / decoded asynchronously.

It reverts https://github.com/emilk/egui/pull/6901 which was my previous
attempt to fix this (but this didn't work since only the tested crate is
compiled with cfg(test) and not it's dependencies)

crates/egui/src/context.rs

@@ -3546,6 +3546,14 @@ impl Context {
     pub fn loaders(&self) -> Arc<Loaders> {
         self.read(|this| this.loaders.clone())
     }
+
+    /// Returns `true` if any image is currently being loaded.
+    pub fn has_pending_images(&self) -> bool {
+        self.read(|this| {
+            this.loaders.image.lock().iter().any(|i| i.has_pending())
+                || this.loaders.bytes.lock().iter().any(|i| i.has_pending())
+        })
+    }
 }
 
 /// ## Viewports

crates/egui/src/load.rs

@@ -328,6 +328,11 @@ pub trait BytesLoader {
 
     /// If the loader caches any data, this should return the size of that cache.
     fn byte_size(&self) -> usize;
+
+    /// Returns `true` if some data is currently being loaded.
+    fn has_pending(&self) -> bool {
+        false
+    }
 }
 
 /// Represents an image which is currently being loaded.
@@ -395,6 +400,13 @@ pub trait ImageLoader {
 
     /// If the loader caches any data, this should return the size of that cache.
     fn byte_size(&self) -> usize;
+
+    /// Returns `true` if some image is currently being loaded.
+    ///  
+    /// NOTE: You probably also want to check [`BytesLoader::has_pending`].
+    fn has_pending(&self) -> bool {
+        false
+    }
 }
 
 /// A texture with a known size.