LibWeb: Fix position: fixed canvases/images disappearing when scrolling

This fixes the Serenity logo vanishing after scrolling on the 4th
birthday post.

The previous check did not account for any translation in the painter.
This now uses the painter's clip rect and translation to work out
if a rect is visible. It also makes use of `absolute_paint_rect()`
rather than `absolute_rect()` which can account for things like
box-shadows.

Userland/Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -37,6 +37,13 @@ void PaintableBox::invalidate_stacking_context()
     m_stacking_context = nullptr;
 }
 
+bool PaintableBox::is_out_of_view(PaintContext& context) const
+{
+    return !enclosing_int_rect(absolute_paint_rect())
+                .translated(context.painter().translation())
+                .intersects(context.painter().clip_rect());
+}
+
 PaintableWithLines::PaintableWithLines(Layout::BlockContainer const& layout_box)
     : PaintableBox(layout_box)
 {