layout: Mark flex items properly during construction

Set the flag of the fragment of children in a flex container according
to the direction of the container. The mark is done on the fragment
because flex item enstablish a stacking context when its z-index is
non-zero ,despite its `position' property.

components/layout/model.rs

@@ -505,7 +505,7 @@ impl ToGfxMatrix for ComputedMatrix {
 }
 
 // Used to specify the logical direction.
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum Direction {
     Inline,
     Block