This is the special data structure required for a masked boolean gadget. a
pointer to this structure must be placed in the gadget's specialinfo field
for a masked boolean gadget.

    struct boolinfo
        {
        UWORD  Flags;
        UWORD  *Mask;
        ULONG  Reserved;
        };

Flags
    Flags must be given the value BOOLMASK.

Mask
    This is a bit mask for highlighting and picking the gadget.
    Construct the mask as a single plane of image data.  The image's
    width and height are determined by the width and height  of the
    gadget's select box.  The mask data must be in Chip memory.

Reserved
    Set this field to NULL.