[subset] Add table duplication overflow resolution.

src/hb-debug.hh

diff --git a/src/hb-debug.hh b/src/hb-debug.hh
index ec3a1ff..a92614d 100644
--- a/src/hb-debug.hh
+++ b/src/hb-debug.hh
@@ -438,6 +438,10 @@
 #define TRACE_SUBSET(this) hb_no_trace_t<bool> trace
 #endif
 
+#ifndef HB_DEBUG_SUBSET_REPACK
+#define HB_DEBUG_SUBSET_REPACK (HB_DEBUG+0)
+#endif
+
 #ifndef HB_DEBUG_DISPATCH
 #define HB_DEBUG_DISPATCH ( \
 	HB_DEBUG_APPLY + \