LibWeb: Move image viewport awareness from ImageBox to ImagePaintable

Images being aware of being visible inside the viewport is a painting
concern, not a layout concern.

Userland/Libraries/LibWeb/Painting/ImagePaintable.h

@@ -6,12 +6,15 @@
 
 #pragma once
 
+#include <LibWeb/HTML/BrowsingContext.h>
 #include <LibWeb/Layout/ImageBox.h>
 #include <LibWeb/Painting/PaintableBox.h>
 
 namespace Web::Painting {
 
-class ImagePaintable final : public PaintableBox {
+class ImagePaintable final
+    : public PaintableBox
+    , public HTML::BrowsingContext::ViewportClient {
     JS_CELL(ImagePaintable, PaintableBox);
 
 public:
@@ -22,6 +25,12 @@ public:
     Layout::ImageBox const& layout_box() const;
 
 private:
+    // ^JS::Cell
+    virtual void finalize() override;
+
+    // ^BrowsingContext::ViewportClient
+    virtual void browsing_context_did_set_viewport_rect(CSSPixelRect const&) final;
+
     ImagePaintable(Layout::ImageBox const&);
 };