[var] Implement DeltaSetIndexMap and advance delta fetching

Not hooked up to hb-ot-font yet.

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

diff --git a/src/hb-ot-layout-common-private.hh b/src/hb-ot-layout-common-private.hh
index 92b5c17..180e5f0 100644
--- a/src/hb-ot-layout-common-private.hh
+++ b/src/hb-ot-layout-common-private.hh
@@ -1323,6 +1323,14 @@
 					     this+regions);
   }
 
+  inline float get_delta (unsigned int index,
+			  int *coords, unsigned int coord_count) const
+  {
+    unsigned int outer = index >> 16;
+    unsigned int inner = index & 0xFFFF;
+    return get_delta (outer, inner, coords, coord_count);
+  }
+
   inline bool sanitize (hb_sanitize_context_t *c) const
   {
     TRACE_SANITIZE (this);