[var] Implement hb_font_get_var_coords_design

Hold design coords and simulate when normalized coords are set directly.

docs/harfbuzz-sections.txt

diff --git a/docs/harfbuzz-sections.txt b/docs/harfbuzz-sections.txt
index 5b2e4b9..0eaae72 100644
--- a/docs/harfbuzz-sections.txt
+++ b/docs/harfbuzz-sections.txt
@@ -346,6 +346,7 @@
 hb_font_get_user_data
 hb_font_get_variation_glyph
 hb_font_get_variation_glyph_func_t
+hb_font_get_var_coords_design
 hb_font_get_var_coords_normalized
 hb_font_glyph_from_string
 hb_font_glyph_to_string