Remove hb_ot_layout_context_t, simplify code

src/hb-ot-layout-common-private.hh

diff --git a/src/hb-ot-layout-common-private.hh b/src/hb-ot-layout-common-private.hh
index 7ef214e..1381964 100644
--- a/src/hb-ot-layout-common-private.hh
+++ b/src/hb-ot-layout-common-private.hh
@@ -526,11 +526,11 @@
 struct Device
 {
 
-  inline hb_position_t get_x_delta (hb_ot_layout_context_t *c) const
-  { return get_delta (c->font->x_ppem, c->font->x_scale); }
+  inline hb_position_t get_x_delta (hb_font_t *font) const
+  { return get_delta (font->x_ppem, font->x_scale); }
 
-  inline hb_position_t get_y_delta (hb_ot_layout_context_t *c) const
-  { return get_delta (c->font->y_ppem, c->font->y_scale); }
+  inline hb_position_t get_y_delta (hb_font_t *font) const
+  { return get_delta (font->y_ppem, font->y_scale); }
 
   inline int get_delta (unsigned int ppem, int scale) const
   {