Add hb_ot_layout_position_finish()

We expect buffer to be setup with default positions before GPOS.

src/hb-buffer.h

diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index 195ad34..fb87220 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -56,16 +56,7 @@
   hb_position_t  y_advance;
   hb_position_t  x_offset;
   hb_position_t  y_offset;
-  /* XXX these should all be replaced by "uint32_t internal" */
-  uint32_t       new_advance :1;	/* if set, the advance width values are
-					   absolute, i.e., they won't be
-					   added to the original glyph's value
-					   but rather replace them */
-  uint32_t       back : 15;		/* number of glyphs to go back
-					   for drawing current glyph */
-  int32_t        cursive_chain : 16;	/* character to which this connects,
-					   may be positive or negative; used
-					   only internally */
+  uint32_t       internal;
 } hb_glyph_position_t;