Just about ready to make Image use shared_ptr.

This lets me get rid of the refcount member and the
ref/deref functions.

src/image.hh

@@ -66,7 +66,7 @@ struct DilloImage {
    float dpi;               /**< Dots per inch */
    int32_t bg_color;        /**< Background color */
    int32_t fg_color;        /**< Foreground color */
-   bitvec_t *BitVec;        /**< Bit vector for decoded rows */
+   std::unique_ptr< bitvec_t > BitVec; /**< Bit vector for decoded rows */
    uint_t ScanNumber;       /**< Current decoding scan */
    ImageState State;        /**< Processing status */