[ft] Add locking FT_Face API

Now that we mutex our internal FT_Face, share it with clients.

New API:
- hb_ft_font_lock_face()
- hb_ft_font_unlock_face()

docs/harfbuzz-sections.txt

diff --git a/docs/harfbuzz-sections.txt b/docs/harfbuzz-sections.txt
index c625b92..93d6de6 100644
--- a/docs/harfbuzz-sections.txt
+++ b/docs/harfbuzz-sections.txt
@@ -365,6 +365,8 @@
 hb_ft_font_create_referenced
 hb_ft_font_changed
 hb_ft_font_get_face
+hb_ft_font_lock_face
+hb_ft_font_unlock_face
 hb_ft_font_set_load_flags
 hb_ft_font_get_load_flags
 hb_ft_font_set_funcs