This method tells a gadget to render itself.  The return value for this
method is not explicitly defined.

GM_RENDER uses a custom message structure (defined in
<intuition/gadgetclass.h>):

    struct gpRender
    {
        ULONG             MethodID;   /* GM_RENDER */
        struct GadgetInfo *gpr_GInfo;
        struct RastPort   *gpr_RPort; /* all ready for use */
        LONG              gpr_Redraw; /* might be a "highlight pass" */
    };

The GM_RENDER message contains a pointer to the Gadget's rastport which it
can use for rendering.  The Gadget renders itself according to how much
imagery it needs to replace.  The gpr_Redraw field contains one of three
values:

GREDRAW_REDRAW  Redraw the entire gadget.

GREDRAW_UPDATE  The user has manipulated the gadget changing the
                imagery.  Update only that part of the gadget's
                imagery that is effected by the user manipulating the
                gadget (for example, the knob and scrolling field of
                the prop gadget).

GREDRAW_TOGGLE  If this gadget supports it, toggle to or from the
                highlighting imagery.