LibWeb: Support CSS property background-clip: text

From https://drafts.csswg.org/css-backgrounds-4/#background-clip
"The background is painted within (clipped to) the intersection of the
border box and the geometry of the text in the element and its in-flow
and floated descendants"

This change implements it in the following way:
1. Traverse the descendants of the element, collecting the Gfx::Path of
   glyphs into a vector.
2. The vector of collected paths is saved in the background painting
   command.
3. The painting commands executor uses the list of glyphs to paint a
   mask for background clipping.

Co-authored-by: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>

Userland/Libraries/LibWeb/Painting/PaintableBox.h

@@ -227,6 +227,8 @@ protected:
     virtual CSSPixelRect compute_absolute_paint_rect() const;
 
 private:
+    Vector<Gfx::Path> compute_text_clip_paths(PaintContext&) const;
+
     [[nodiscard]] virtual bool is_paintable_box() const final { return true; }
 
     enum class ScrollDirection {