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/Forward.h

@@ -60,7 +60,7 @@ class Node;
 class Origin;
 class Page;
 class PageClient;
-class RenderingContext;
+class PaintContext;
 class Resource;
 class ResourceLoader;
 class Selector;