+/** Construct a subtitle, which is an image and a position.
+ * @param p Position within the (uncropped) source frame.
+ * @param i Image of the subtitle (should be RGBA).
+ */
+Subtitle::Subtitle (Position p, shared_ptr<Image> i)
+ : _position (p)
+ , _image (i)
+{
+
+}
+
+/** Given the area of a subtitle, work out the area it should
+ * take up when its video frame is scaled up, and it is optionally
+ * itself scaled and offset.
+ * @param target_x_scale the x scaling of the video frame that the subtitle is in.
+ * @param target_y_scale the y scaling of the video frame that the subtitle is in.
+ * @param sub_area The area of the subtitle within the original source.
+ * @param subtitle_offset y offset to apply to the subtitle position (+ve is down)
+ * in the coordinate space of the source.
+ * @param subtitle_scale scaling factor to apply to the subtitle image.
+ */
+Rect
+subtitle_transformed_area (