LibWeb: Cache width of "alt" text in ImageBox

We were constantly measuring and re-measuring the "alt" attribute text
of ImageBox layout nodes, even when the alt text didn't change. By
caching this, we avoid a *lot* of repeated text measurement work.

Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp

@@ -78,6 +78,11 @@ void HTMLImageElement::parse_attribute(FlyString const& name, String const& valu
 
     if (name == HTML::AttributeNames::src && !value.is_empty())
         m_image_loader.load(document().parse_url(value));
+
+    if (name == HTML::AttributeNames::alt) {
+        if (layout_node())
+            verify_cast<Layout::ImageBox>(*layout_node()).dom_node_did_update_alt_text({});
+    }
 }
 
 RefPtr<Layout::Node> HTMLImageElement::create_layout_node(NonnullRefPtr<CSS::StyleProperties> style)

Userland/Libraries/LibWeb/Layout/ImageBox.cpp

@@ -59,7 +59,13 @@ void ImageBox::prepare_for_replaced_layout()
         auto alt = image_element.alt();
         if (alt.is_empty())
             alt = image_element.src();
-        set_intrinsic_width(font.width(alt) + 16);
+
+        float alt_text_width = 0;
+        if (!m_cached_alt_text_width.has_value())
+            m_cached_alt_text_width = font.width(alt);
+        alt_text_width = m_cached_alt_text_width.value();
+
+        set_intrinsic_width(alt_text_width + 16);
         set_intrinsic_height(font.pixel_size() + 16);
     }
 
@@ -68,6 +74,11 @@ void ImageBox::prepare_for_replaced_layout()
     }
 }
 
+void ImageBox::dom_node_did_update_alt_text(Badge<HTML::HTMLImageElement>)
+{
+    m_cached_alt_text_width = {};
+}
+
 bool ImageBox::renders_as_alt_text() const
 {
     if (is<HTML::HTMLImageElement>(dom_node()))

Userland/Libraries/LibWeb/Layout/ImageBox.h

@@ -29,6 +29,8 @@ public:
 
     auto const& image_loader() const { return m_image_loader; }
 
+    void dom_node_did_update_alt_text(Badge<HTML::HTMLImageElement>);
+
 private:
     // ^BrowsingContext::ViewportClient
     virtual void browsing_context_did_set_viewport_rect(Gfx::IntRect const&) final;
@@ -37,6 +39,8 @@ private:
     int preferred_height() const;
 
     ImageLoader const& m_image_loader;
+
+    Optional<float> m_cached_alt_text_width;
 };
 
 }