Add hb_buffer_normalize_glyphs() and hb-shape --normalize-glyphs

This reorders glyphs within the cluster to a nominal order.  This should
have no visible effect on the output, but helps with testing, for
getting the same hb-shape output for visually-equal glyphs for each
cluster.
diff --git a/util/options.cc b/util/options.cc
index db1b244..584190e 100644
--- a/util/options.cc
+++ b/util/options.cc
@@ -396,6 +396,7 @@
     {"language",	0, 0, G_OPTION_ARG_STRING,	&this->language,		"Set text language (default: $LANG)",	"langstr"},
     {"script",		0, 0, G_OPTION_ARG_STRING,	&this->script,			"Set text script (default: auto)",	"ISO-15924 tag"},
     {"utf8-clusters",	0, 0, G_OPTION_ARG_NONE,	&this->utf8_clusters,		"Use UTF8 byte indices, not char indices",	NULL},
+    {"normalize-glyphs",0, 0, G_OPTION_ARG_NONE,	&this->normalize_glyphs,	"Rearrange glyph clusters in nominal order",	NULL},
     {NULL}
   };
   parser->add_group (entries,