FIX: Ensure loading thumbnails are used in Safari (#18204)

In Safari, `img.complete` is sometimes true even before the image is loaded. Checking for the presence of `img.naturalHeight` seems to be more reliable. It is very difficult to write a test for this behaviour due to the dependence on network conditions, scroll location, etc.

`img.naturalHeight` is supported by all our target browsers, and I have verified the functionality of this commit in Chrome, Safari and Firefox.

app/assets/javascripts/discourse/app/lib/lazy-load-images.js

@@ -9,13 +9,19 @@ function forEachImage(post, callback) {
   });
 }
 
+function isLoaded(img) {
+  // In Safari, img.complete sometimes returns true even when the image is not loaded.
+  // naturalHeight seems to be a more reliable check
+  return !!img.naturalHeight;
+}
+
 export function nativeLazyLoading(api) {
   api.decorateCookedElement(
     (post) =>
       forEachImage(post, (img) => {
         img.loading = "lazy";
         if (img.dataset.smallUpload) {
-          if (!img.complete) {
+          if (!isLoaded(img)) {
             if (!img.onload) {
               img.onload = () => {
                 img.style.removeProperty("background-image");