Add hb_font_set_parent()

No reason to not have it.  Makes life easier later.
We (hb-ft, hb-ot-font, etc) can use this API to inject new
parent into a font...

src/hb-font.cc

@@ -1058,6 +1058,32 @@ hb_font_is_immutable (hb_font_t *font)
   return font->immutable;
 }
 
+/**
+ * hb_font_set_parent:
+ * @font: a font.
+ * @parent: new parent.
+ *
+ * Sets parent font of @font.
+ *
+ * Since: 1.0.5
+ **/
+void
+hb_font_set_parent (hb_font_t *font,
+		    hb_font_t *parent)
+{
+  if (font->immutable)
+    return;
+
+  if (!parent)
+    parent = hb_font_get_empty ();
+
+  hb_font_t *old = font->parent;
+
+  font->parent = hb_font_reference (parent);
+
+  hb_font_destroy (old);
+}
+
 /**
  * hb_font_get_parent:
  * @font: a font.

src/hb-font.h

@@ -459,6 +459,10 @@ hb_font_make_immutable (hb_font_t *font);
 hb_bool_t
 hb_font_is_immutable (hb_font_t *font);
 
+void
+hb_font_set_parent (hb_font_t *font,
+		    hb_font_t *parent);
+
 hb_font_t *
 hb_font_get_parent (hb_font_t *font);
 

test/api/test-font.c

@@ -387,6 +387,18 @@ test_font_properties (void)
 
   g_assert (hb_font_get_face (font) == face);
   g_assert (hb_font_get_parent (font) == NULL);
+  subfont = hb_font_create_sub_font (font);
+  g_assert (hb_font_get_parent (subfont) == font);
+  hb_font_set_parent(subfont, NULL);
+  g_assert (hb_font_get_parent (subfont) == hb_font_get_empty());
+  hb_font_set_parent(subfont, font);
+  g_assert (hb_font_get_parent (subfont) == font);
+  hb_font_set_parent(subfont, NULL);
+  hb_font_make_immutable (subfont);
+  g_assert (hb_font_get_parent (subfont) == hb_font_get_empty());
+  hb_font_set_parent(subfont, font);
+  g_assert (hb_font_get_parent (subfont) == hb_font_get_empty());
+  hb_font_destroy (subfont);
 
 
   /* Check scale */