Bug 1680387 - Fix interaction with src-set() / image-set(), and enable the feature by default. r=tnikkel

As discussed here: https://github.com/whatwg/html/pull/5574#issuecomment-826347560

This matches other browsers.

Depends on D113265

Differential Revision: https://phabricator.services.mozilla.com/D113267

dom/html/HTMLImageElement.cpp

@@ -732,7 +732,7 @@ nsIntSize HTMLImageElement::NaturalSize() {
   if (mResponsiveSelector) {
     float density = mResponsiveSelector->GetSelectedImageDensity();
     MOZ_ASSERT(density >= 0.0);
-    resolution = {density, density};
+    resolution.ScaleBy(density);
   }
 
   resolution.ApplyTo(size.width, size.height);