Rename invisible_codepoint to invisible_glyph in API

Deleted recently added API:
    hb_buffer_set_invisible_codepoint()
    hb_buffer_get_invisible_codepoint()

    hb-shape / hb-view --invisible-codepoint

New API:
    hb_buffer_set_invisible_glyph()
    hb_buffer_get_invisible_glyph()

    hb-shape / hb-view --invisible-glyph

Fixes https://github.com/harfbuzz/harfbuzz/issues/1216
diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index d99ae7d..d0aed02 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -342,11 +342,11 @@
 hb_buffer_get_replacement_codepoint (hb_buffer_t    *buffer);
 
 HB_EXTERN void
-hb_buffer_set_invisible_codepoint (hb_buffer_t    *buffer,
-				   hb_codepoint_t  invisible);
+hb_buffer_set_invisible_glyph (hb_buffer_t    *buffer,
+			       hb_codepoint_t  invisible);
 
 HB_EXTERN hb_codepoint_t
-hb_buffer_get_invisible_codepoint (hb_buffer_t    *buffer);
+hb_buffer_get_invisible_glyph (hb_buffer_t    *buffer);
 
 
 HB_EXTERN void