Improve Subst/Pos SubTable access and sanitize

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

diff --git a/src/hb-ot-layout-gpos-private.hh b/src/hb-ot-layout-gpos-private.hh
index bebde66..1454b5b 100644
--- a/src/hb-ot-layout-gpos-private.hh
+++ b/src/hb-ot-layout-gpos-private.hh
@@ -1440,7 +1440,7 @@
 struct PosLookup : Lookup
 {
   inline const PosLookupSubTable& get_subtable (unsigned int i) const
-  { return (const PosLookupSubTable&) Lookup::get_subtable (i); }
+  { return this+CONST_CAST (OffsetArrayOf<PosLookupSubTable>, subTable, 0)[i]; }
 
   /* Like get_type(), but looks through extension lookups.
    * Never returns Extension */
@@ -1519,7 +1519,7 @@
   inline bool sanitize (SANITIZE_ARG_DEF) {
     TRACE_SANITIZE ();
     if (!Lookup::sanitize (SANITIZE_ARG)) return false;
-    OffsetArrayOf<PosLookupSubTable> &list = (OffsetArrayOf<PosLookupSubTable> &) subTable;
+    OffsetArrayOf<PosLookupSubTable> &list = CAST (OffsetArrayOf<PosLookupSubTable>, subTable, 0);
     return SANITIZE_THIS (list);
   }
 };

src/hb-ot-layout-gsub-private.hh

diff --git a/src/hb-ot-layout-gsub-private.hh b/src/hb-ot-layout-gsub-private.hh
index 1a49edb..d04ea99 100644
--- a/src/hb-ot-layout-gsub-private.hh
+++ b/src/hb-ot-layout-gsub-private.hh
@@ -736,7 +736,7 @@
 struct SubstLookup : Lookup
 {
   inline const SubstLookupSubTable& get_subtable (unsigned int i) const
-  { return CONST_CAST (SubstLookupSubTable, Lookup::get_subtable (i), 0); }
+  { return this+CONST_CAST (OffsetArrayOf<SubstLookupSubTable>, subTable, 0)[i]; }
 
   /* Like get_type(), but looks through extension lookups.
    * Never returns Extension */
@@ -829,7 +829,7 @@
   inline bool sanitize (SANITIZE_ARG_DEF) {
     TRACE_SANITIZE ();
     if (!Lookup::sanitize (SANITIZE_ARG)) return false;
-    OffsetArrayOf<SubstLookupSubTable> &list = (OffsetArrayOf<SubstLookupSubTable> &) subTable;
+    OffsetArrayOf<SubstLookupSubTable> &list = CAST (OffsetArrayOf<SubstLookupSubTable>, subTable, 0);
     return SANITIZE_THIS (list);
   }
 };