add GL.draw_text and Widget#draw_text

runtime/lib/widget.rb

@@ -34,4 +34,8 @@ module Widget
   def draw_rect(x, y, width, height, r, g, b, a)
     GL.draw_rect(@x + x, @y + y, width, height, r, g, b, a)
   end
+
+  def draw_text(font, x, y, string, r, g, b, a)
+    GL.draw_text(font, @x + x, @y + y, string, r, g, b, a)
+  end
 end