The GRUP chunk combines several DR2D objects into one.  This chunk is
only valid inside nested DR2D FORMs, and must be the first chunk in
the FORM.

    struct GROUPstruct {
        ULONG       ID;
        ULONG       Size;
        USHORT      NumObjs;
    };

The NumObjs field contains the number of objects contained in this
group.  Note that the layer of the GRUP FORM overrides the layer of
objects within the GRUP.  The following example illustrates the layout
of the GRUP (and FILL) chunk.

   FORM { DR2D              /* Top-level drawing... */
            DRHD { ... }    /* Confirmed by presence of DRHD chunk */
            CMAP { ... }    /* Various other things... */
            FONS { ... }
            FORM { DR2D             /* A nested form... */
                    FILL { 1 }      /* Ah!  The fill-pattern table */
                    CPLY { ... }    /* with only 1 object */
            }
            FORM { DR2D             /* Yet another nested form */
                    GRUP { ..., 3 } /* Ah! A group of 3 objects */
                    TEXT { ... }
                    CPLY { ... }
                    OPLY { ... }
            }
            FORM { DR2D             /* Still another nested form */
                    GRUP { ..., 2 } /* A GRUP with 2 objects */
                    OPLY { ... }
                    TEXT { ... }
            }
    }