Add API/cmdline to show glyph extents when serializing buffer

New API: HB_BUFFER_SERIALIZE_FLAG_GLYPH_EXTENTS

hb-shape now accepts --show-extents.

Patch from Simon Cozens.

util/hb-shape.cc

diff --git a/util/hb-shape.cc b/util/hb-shape.cc
index f38f387..3bd2184 100644
--- a/util/hb-shape.cc
+++ b/util/hb-shape.cc
@@ -70,6 +70,8 @@
       flags |= HB_BUFFER_SERIALIZE_FLAG_NO_CLUSTERS;
     if (!format.show_positions)
       flags |= HB_BUFFER_SERIALIZE_FLAG_NO_POSITIONS;
+    if (format.show_extents)
+      flags |= HB_BUFFER_SERIALIZE_FLAG_GLYPH_EXTENTS;
     format_flags = (hb_buffer_serialize_flags_t) flags;
   }
   void new_line (void)