[dwrite] Provide an experimental API for justification (#279)

src/hb-directwrite.h

diff --git a/src/hb-directwrite.h b/src/hb-directwrite.h
index 0e1b479..0eb116f 100644
--- a/src/hb-directwrite.h
+++ b/src/hb-directwrite.h
@@ -29,6 +29,10 @@
 
 HB_BEGIN_DECLS
 
+HB_EXTERN hb_bool_t
+hb_shape_dwrite_experimental_width(hb_font_t *font, hb_buffer_t *buffer,
+  const hb_feature_t *features, unsigned int num_features, float width);
+
 HB_END_DECLS
 
 #endif /* HB_DIRECTWRITE_H */