PERF: Improve lazy-load performance in Safari

Safari starts loading images as soon as attributes are modified. Modern browsers all prefer the srcset attribute over src, so we should remove srcset last, and add it first.

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

@@ -22,9 +22,10 @@ function hide(image) {
     height: image.height,
     className: image.className
   });
-  image.removeAttribute("srcset");
 
   image.src = image.dataset.smallUpload || LOADING_DATA;
+  image.removeAttribute("srcset");
+
   image.removeAttribute("data-small-upload");
 }
 
@@ -35,10 +36,10 @@ function show(image) {
   if (imageData) {
     const copyImg = new Image();
     copyImg.onload = () => {
-      image.src = copyImg.src;
       if (copyImg.srcset) {
         image.srcset = copyImg.srcset;
       }
+      image.src = copyImg.src;
       image.classList.remove("d-lazyload-hidden");
 
       if (image.onload) {
@@ -55,12 +56,12 @@ function show(image) {
       copyImg.onload = null;
     };
 
-    copyImg.src = imageData.src;
-
     if (imageData.srcset) {
       copyImg.srcset = imageData.srcset;
     }
 
+    copyImg.src = imageData.src;
+
     // width of image may not match, use computed style which
     // is the actual size of the image
     const computedStyle = window.getComputedStyle(image);