Don't add reference in get_unicode_funcs() functions

Users don't expect a new reference returned from a get() function.
Indeed, all users of that API I foud where NOT destroying the reference.
Just change the implementations to NOT return a reference.

This applies to the following APIs:
hb_unicode_funcs_get_default()
hb_glib_get_unicode_funcs()
hb_icu_get_unicode_funcs()

Fixes https://github.com/harfbuzz/harfbuzz/issues/1134

src/hb-buffer.cc

@@ -216,7 +216,7 @@ hb_buffer_t::reset (void)
     return;
 
   hb_unicode_funcs_destroy (unicode);
-  unicode = hb_unicode_funcs_get_default ();
+  unicode = hb_unicode_funcs_reference (hb_unicode_funcs_get_default ());
   flags = HB_BUFFER_FLAG_DEFAULT;
   replacement = HB_BUFFER_REPLACEMENT_CODEPOINT_DEFAULT;
 
@@ -908,7 +908,6 @@ hb_buffer_set_unicode_funcs (hb_buffer_t        *buffer,
   if (!unicode_funcs)
     unicode_funcs = hb_unicode_funcs_get_default ();
 
-
   hb_unicode_funcs_reference (unicode_funcs);
   hb_unicode_funcs_destroy (buffer->unicode);
   buffer->unicode = unicode_funcs;

src/hb-glib.cc

@@ -401,7 +401,7 @@ void free_static_glib_funcs (void)
 hb_unicode_funcs_t *
 hb_glib_get_unicode_funcs (void)
 {
-  return hb_unicode_funcs_reference (static_glib_funcs.get_unconst ());
+  return static_glib_funcs.get_unconst ();
 }
 
 

src/hb-icu.cc

@@ -390,5 +390,5 @@ void free_static_icu_funcs (void)
 hb_unicode_funcs_t *
 hb_icu_get_unicode_funcs (void)
 {
-  return hb_unicode_funcs_reference (static_icu_funcs.get_unconst ());
+  return static_icu_funcs.get_unconst ();
 }

src/hb-ucdn.cc

@@ -275,5 +275,5 @@ extern "C" HB_INTERNAL
 hb_unicode_funcs_t *
 hb_ucdn_get_unicode_funcs (void)
 {
-  return hb_unicode_funcs_reference (static_ucdn_funcs.get_unconst ());
+  return static_ucdn_funcs.get_unconst ();
 }