[subset] Prepare the repacker for handling 24bit offsets in GSUB/GPOS.

The boring expansion (https://github.com/be-fonts/boring-expansion-spec) plans to introduce 24bit offsets into GSUB/GPOS. This changes the repacker to treat 24 bit offsets similar to 32 bit offsets and assign the top level 24 bit offsets into spaces to improve packing.

src/hb-repacker.hh

diff --git a/src/hb-repacker.hh b/src/hb-repacker.hh
index 683a441..57c0631 100644
--- a/src/hb-repacker.hh
+++ b/src/hb-repacker.hh
@@ -172,7 +172,7 @@
       && will_overflow)
   {
     DEBUG_MSG (SUBSET_REPACK, nullptr, "Assigning spaces to 32 bit subgraphs.");
-    if (sorted_graph.assign_32bit_spaces ())
+    if (sorted_graph.assign_spaces ())
       sorted_graph.sort_shortest_distance ();
   }