Prevent multiple notifications for image dimensions (#36600)

Added a simple check to only perform metadata extraction and listener
notification when we haven't already processed the metadata for an image

Testing: Existing tests should cover if we break decoding image metadata
complete.
Fixes: #36502

---------

Signed-off-by: Barigbue <barigbuenbira@gmail.com>
Co-authored-by: Josh Matthews <josh@joshmatthews.net>

components/net/image_cache.rs

@@ -608,6 +608,7 @@ impl ImageCache for ImageCacheImpl {
                 pending_load.bytes.extend_from_slice(&data);
 
                 //jmr0 TODO: possibly move to another task?
+                if pending_load.metadata.is_none() {
                     let mut reader = std::io::Cursor::new(pending_load.bytes.as_slice());
                     if let Ok(info) = imsz_from_reader(&mut reader) {
                         let img_metadata = ImageMetadata {
@@ -619,6 +620,7 @@ impl ImageCache for ImageCacheImpl {
                         }
                         pending_load.metadata = Some(img_metadata);
                     }
+                }
             },
             (FetchResponseMsg::ProcessResponseEOF(_, result), key) => {
                 debug!("Received EOF for {:?}", key);