Remove NullPool visibility hack

We keep a separate private NullPool in libharfbuzz-subset.so
This can fire back later since now there are two separate null objects,
and comparisons to the null pool can be confused.  Something to deal
with when it comes up. Ouch!

src/hb-open-type-private.hh

@@ -134,9 +134,7 @@ static inline Type& StructAfter(TObject &X)
 
 #define HB_NULL_POOL_SIZE 264
 static_assert (HB_NULL_POOL_SIZE % sizeof (void *) == 0, "Align HB_NULL_POOL_SIZE.");
-// TODO add back HB_INTERNAL once we figure out how to not have that break
-// subset compile
-extern const void * const _hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)];
+extern HB_INTERNAL const void * const _hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)];
 
 /* Generic nul-content Null objects. */
 template <typename Type>

src/hb-subset.cc

@@ -34,6 +34,10 @@
 
 #include "hb-ot-glyf-table.hh"
 
+
+const void * const OT::_hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)] = {};
+
+
 struct hb_subset_profile_t {
   hb_object_header_t header;
   ASSERT_POD ();