layout: Explicitly thread border box dimensions and relative offsets

through display list building.

The old `flow_origin` concept was ill-defined (sometimes the border box
plus the flow origin, sometimes including horizontal margins and
sometimes not, sometimes including relative position and sometimes not),
leading to brittleness and test failures. This commit reworks the logic
to always pass border box origins in during display list building.

components/layout/table_cell.rs

@@ -9,13 +9,13 @@
 use block::{BlockFlow, ISizeAndMarginsComputer, MarginsMayCollapseFlag};
 use context::LayoutContext;
 use flow::{Flow, FlowClass};
-use fragment::{Fragment, FragmentOverflowIterator};
+use fragment::{Fragment, FragmentBorderBoxIterator};
 use model::{MaybeAuto};
 use layout_debug;
 use table::InternalTable;
 use wrapper::ThreadSafeLayoutNode;
 
-use geom::Rect;
+use geom::{Point2D, Rect};
 use servo_util::geometry::Au;
 use std::fmt;
 use style::{UnsignedIntegerAttribute, ComputedValues};
@@ -167,8 +167,10 @@ impl Flow for TableCellFlow {
         self.block_flow.compute_overflow()
     }
 
-    fn iterate_through_fragment_overflow(&self, iterator: &mut FragmentOverflowIterator) {
-        self.block_flow.iterate_through_fragment_overflow(iterator);
+    fn iterate_through_fragment_border_boxes(&self,
+                                             iterator: &mut FragmentBorderBoxIterator,
+                                             stacking_context_position: &Point2D<Au>) {
+        self.block_flow.iterate_through_fragment_border_boxes(iterator, stacking_context_position)
     }
 }