Fixed incorrect comments and names.

Fixed a comment made useless by the introduction of the Point class. Fixed the incorrect comment in Surface::render saying that dst_xy's width and height fields were unused. Changed dst_xy name to dst_rect so that it is more explicit.
Actually, I modified this function in the original point_and_size branch. The comment has been wrong for some commits already. The functions now takes a Point, so naturally the width and height are ignored, but it is useless to say it in the comments anymore.