[GPOS] Rewrite cursive attachment slightly differently

In anticipation for upcoming fix for bug reported by
Khaled in thread "Issue with cursive attachment".

src/hb-ot-layout-gpos-table.hh

diff --git a/src/hb-ot-layout-gpos-table.hh b/src/hb-ot-layout-gpos-table.hh
index da9506c..6dcada6 100644
--- a/src/hb-ot-layout-gpos-table.hh
+++ b/src/hb-ot-layout-gpos-table.hh
@@ -960,20 +960,32 @@
     }
 
     /* Cross-direction adjustment */
-    if  (c->lookup_props & LookupFlag::RightToLeft) {
-      pos[i].cursive_chain() = j - i;
-      if (likely (HB_DIRECTION_IS_HORIZONTAL (c->direction)))
-	pos[i].y_offset = entry_y - exit_y;
-      else
-	pos[i].x_offset = entry_x - exit_x;
-    } else {
-      pos[j].cursive_chain() = i - j;
-      if (likely (HB_DIRECTION_IS_HORIZONTAL (c->direction)))
-	pos[j].y_offset = exit_y - entry_y;
-      else
-	pos[j].x_offset = exit_x - entry_x;
+
+    /* We attach child to parent (think graph theory and rooted trees whereas
+     * the root stays on baseline and each node aligns itself against its
+     * parent.
+     *
+     * Optimize things for the case of RightToLeft, as that's most common in
+     * Arabinc. */
+    unsigned int child  = i;
+    unsigned int parent = j;
+    hb_position_t x_offset = entry_x - exit_x;
+    hb_position_t y_offset = entry_y - exit_y;
+    if  (!(c->lookup_props & LookupFlag::RightToLeft))
+    {
+      unsigned int k = child;
+      child = parent;
+      parent = k;
+      x_offset = -x_offset;
+      y_offset = -y_offset;
     }
 
+    pos[child].cursive_chain() = parent - child;
+    if (likely (HB_DIRECTION_IS_HORIZONTAL (c->direction)))
+      pos[child].y_offset = y_offset;
+    else
+      pos[child].x_offset = x_offset;
+
     buffer->idx = j;
     return TRACE_RETURN (true);
   }