LibWeb: Compute the final border-style property before painting

Instead of doing a CSS property lookup for the line style of each
border edge during paint, we now cache the final CSS::LineStyle to use
in the Layout::BorderData.

Libraries/LibWeb/Layout/LayoutStyle.h

@@ -41,6 +41,7 @@ public:
 struct BorderData {
 public:
     Color color { Color::Transparent };
+    CSS::LineStyle line_style { CSS::LineStyle::None };
     float width { 0 };
 };