[ft] Add hb_ft_font_changed()

When the font size or variations settings on underlying FT_Face change,
one can call hb_ft_font_changed() and continue using hb_font created using
hb_ft_font_create().

Fixes https://github.com/behdad/harfbuzz/issues/559

New API:
hb_ft_font_changed()

docs/harfbuzz-sections.txt

diff --git a/docs/harfbuzz-sections.txt b/docs/harfbuzz-sections.txt
index 6d86d94..06a78f5 100644
--- a/docs/harfbuzz-sections.txt
+++ b/docs/harfbuzz-sections.txt
@@ -291,6 +291,7 @@
 hb_ft_face_create_referenced
 hb_ft_font_create
 hb_ft_font_create_referenced
+hb_ft_font_changed
 hb_ft_font_get_face
 hb_ft_font_set_load_flags
 hb_ft_font_get_load_flags