LibWeb: Don't force HTMLImageElement to have a legacy ImageLoader

We achieve this by adding a new Layout::ImageProvider class and having
both HTMLImageElement and HTMLObjectElement inherit from it.

The HTML spec is vague on how object image loading should work, which
is why this first pass is focusing on image elements.

Userland/Libraries/LibWeb/Layout/ImageProvider.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Web::Layout {
+
+class ImageProvider {
+public:
+    virtual ~ImageProvider() { }
+
+    virtual RefPtr<Gfx::Bitmap const> current_image_bitmap() const = 0;
+    virtual void set_visible_in_viewport(bool) = 0;
+};
+
+}