LibWeb: Attach BackgroundAttachment::Fixed to the window view port

This fixes the placement of several background images on Acid2, most
notably the background of the eyes and the red rectangle near the bottom
of the head.

Userland/Libraries/LibWeb/Painting/BackgroundPainting.cpp

@@ -6,12 +6,14 @@
  */
 
 #include <LibGfx/Painter.h>
+#include <LibWeb/Layout/InitialContainingBlock.h>
 #include <LibWeb/Layout/Node.h>
 #include <LibWeb/Painting/BackgroundPainting.h>
 #include <LibWeb/Painting/PaintContext.h>
 
 namespace Web::Painting {
 
+// https://www.w3.org/TR/css-backgrounds-3/#backgrounds
 void paint_background(PaintContext& context, Layout::NodeWithStyleAndBoxModelMetrics const& layout_node, Gfx::IntRect const& border_rect, Color background_color, Vector<CSS::BackgroundLayerData> const* background_layers, BorderRadiusData const& border_radius)
 {
     auto& painter = context.painter();
@@ -57,10 +59,18 @@ void paint_background(PaintContext& context, Layout::NodeWithStyleAndBoxModelMet
         painter.save();
         painter.add_clip_rect(clip_rect);
 
-        // FIXME: Attachment
+        Gfx::IntRect background_positioning_area;
 
-        // Origin
-        auto background_positioning_area = get_box(layer.origin);
+        // Attachment and Origin
+        switch (layer.attachment) {
+        case CSS::BackgroundAttachment::Fixed:
+            background_positioning_area = layout_node.root().browsing_context().viewport_rect();
+            break;
+        case CSS::BackgroundAttachment::Local:
+        case CSS::BackgroundAttachment::Scroll:
+            background_positioning_area = get_box(layer.origin);
+            break;
+        }
 
         // Size
         Gfx::IntRect image_rect;