Add get_nominal_glyph() and get_variation_glyph() instead of get_glyph()

New API:
- hb_font_get_nominal_glyph_func_t
- hb_font_get_variation_glyph_func_t
- hb_font_funcs_set_nominal_glyph_func()
- hb_font_funcs_set_variation_glyph_func()
- hb_font_get_nominal_glyph()
- hb_font_get_variation_glyph()

Deprecated API:
- hb_font_get_glyph_func_t
- hb_font_funcs_set_glyph_func()

Clients that implement their own font-funcs are encouraged to replace
their get_glyph() implementation with a get_nominal_glyph() and
get_variation_glyph() pair.  The variation version can assume that
variation_selector argument is not zero.

src/hb-ot-shape-complex-use.cc

diff --git a/src/hb-ot-shape-complex-use.cc b/src/hb-ot-shape-complex-use.cc
index 6fbfe2b..d2b6e63 100644
--- a/src/hb-ot-shape-complex-use.cc
+++ b/src/hb-ot-shape-complex-use.cc
@@ -499,7 +499,7 @@
     return;
 
   hb_glyph_info_t dottedcircle = {0};
-  if (!font->get_glyph (0x25CCu, 0, &dottedcircle.codepoint))
+  if (!font->get_nominal_glyph (0x25CCu, &dottedcircle.codepoint))
     return;
   dottedcircle.use_category() = hb_use_get_categories (0x25CC);