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>

Tests/LibWeb/Ref/reference/css-background-clip-text-ref.html

@@ -0,0 +1,15 @@
+<style>
+  * {
+    margin: 0;
+  }
+  body {
+    background-color: white;
+  }
+</style>
+<!-- To rebase:
+  1. Open background-clip-text.html in Ladybird
+  2. Resize the window just above the width of the canvas
+  3. Right click > "Take Full Screenshot"
+  4. Update the image below:
+-->
+<img src="./images/css-background-clip-text.png">