LibWeb: Special case SVG masks during layout

Rather than try to lay out masks normally, this updates the TreeBuilder
to create layout nodes for masks as a child of their user (i.e. the
masked element). This allows each use of a mask to be laid out
differently, which makes supporting `maskContentUnits=objectBoundingBox`
fairly easy.

The `SVGFormattingContext` is then updated to lay out masks last (as
their sizing may depend on their parent), and treats them like
viewports.

This is pretty ad-hoc, but the SVG specification does not give any
guidance on how to actually implement this.

Userland/Libraries/LibWeb/Layout/SVGMaskBox.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2024, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Layout/SVGMaskBox.h>
+#include <LibWeb/Painting/SVGMaskPaintable.h>
+#include <LibWeb/Painting/StackingContext.h>
+
+namespace Web::Layout {
+
+SVGMaskBox::SVGMaskBox(DOM::Document& document, SVG::SVGMaskElement& element, NonnullRefPtr<CSS::StyleProperties> properties)
+    : SVGGraphicsBox(document, element, properties)
+{
+}
+
+JS::GCPtr<Painting::Paintable> SVGMaskBox::create_paintable() const
+{
+    return Painting::SVGMaskPaintable::create(*this);
+}
+
+}