Make it an error to include non-top-level headers

Users should #include <hb.h> (or hb-ft.h, hb-glib.h, etc), but
never things like hb-shape.h directly.  This makes it easier to
refactor headers later on without breaking compatibility.

src/hb-graphite2.h

diff --git a/src/hb-graphite2.h b/src/hb-graphite2.h
index 68bd019..3eba9e0 100644
--- a/src/hb-graphite2.h
+++ b/src/hb-graphite2.h
@@ -26,8 +26,7 @@
 #ifndef HB_GRAPHITE2_H
 #define HB_GRAPHITE2_H
 
-#include "hb-common.h"
-#include "hb-shape.h"
+#include "hb.h"
 
 HB_BEGIN_DECLS
 
@@ -41,6 +40,8 @@
 		   unsigned int        num_features,
 		   const char * const *shaper_options);
 
+/* TODO add gr_font/face etc getters and other glue API */
+
 HB_END_DECLS
 
 #endif /* HB_GRAPHITE2_H */