Add a prototype of DirectWrite as a shaping backend

src/hb-directwrite.h

+/*
+ * Copyright © 2015  Ebrahim Byagowi
+ *
+ *  This is part of HarfBuzz, a text shaping library.
+ *
+ * Permission is hereby granted, without written agreement and without
+ * license or royalty fees, to use, copy, modify, and distribute this
+ * software and its documentation for any purpose, provided that the
+ * above copyright notice and the following two paragraphs appear in
+ * all copies of this software.
+ *
+ * IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR
+ * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
+ * ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN
+ * IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
+ * DAMAGE.
+ *
+ * THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
+ * BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
+ * ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO
+ * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
+ */
+
+#ifndef HB_DIRECTWRITE_H
+#define HB_DIRECTWRITE_H
+
+#include "hb.h"
+
+HB_BEGIN_DECLS
+
+HB_END_DECLS
+
+#endif /* HB_UNISCRIBE_H */

src/hb-shaper-list.hh

@@ -46,6 +46,9 @@ HB_SHAPER_IMPLEMENT (ot) /* <--- This is our main OpenType shaper. */
 #ifdef HAVE_UNISCRIBE
 HB_SHAPER_IMPLEMENT (uniscribe)
 #endif
+#ifdef HAVE_DIRECTWRITE
+HB_SHAPER_IMPLEMENT (directwrite)
+#endif
 #ifdef HAVE_CORETEXT
 HB_SHAPER_IMPLEMENT (coretext)
 #endif