gfx: Box stacking contexts to minimize memmove traffic.

`memmove` was showing up high in the profile when concatenating and
shorting display lists. This change drastically reduces the `memmove`
cost in exchange for some minor additional allocation cost.

components/layout/table_wrapper.rs

@@ -451,7 +451,7 @@ impl Flow for TableWrapperFlow {
 
     fn collect_stacking_contexts(&mut self,
                                  parent_id: StackingContextId,
-                                 contexts: &mut Vec<StackingContext>)
+                                 contexts: &mut Vec<Box<StackingContext>>)
                                  -> StackingContextId {
         self.block_flow.collect_stacking_contexts(parent_id, contexts)
     }