LibWeb: Rename LayoutNode::render() to paint()

"Paint" matches what we call this in the rest of the system. Let's not
confuse things by mixing paint/render/draw all the time. I'm guilty of
this in more places..

Also rename RenderingContext => PaintContext.

Libraries/LibWeb/Layout/LineBoxFragment.h

@@ -60,7 +60,7 @@ public:
 
     float absolute_x() const { return absolute_rect().x(); }
 
-    void render(RenderingContext&);
+    void render(PaintContext&);
 
     bool ends_in_whitespace() const;
     bool is_justifiable_whitespace() const;