Virtual member functions

• virtual void prepare(DisplayDevice *) - goes through all the registered objects and prepares them for drawing. This should be called before draw.
• virtual void draw(DisplayDevice *) - actually draws the scene, using the given DisplayDevice. If this display is in stereo, this will first draw the left eye view, then the right.
• virtual int filedraw(char *, char *, DisplayDevice *) - very similar to draw, except this draws the scene to a file-based DisplayDevice. The first two arguments are the format, and the filename. The different formats and file-rendering objects are maintained by a global instance of the object FileRenderList.