Allow several repeated calls to opj_set_decode_area() and opj_decode() for single...

Allow several repeated calls to opj_set_decode_area() and opj_decode() for single-tiled images

* Only works for single-tiled images --> will error out cleanly, as currently
  in other cases
* Save re-reading the codestream for the tile, and re-use code-blocks of the
  previous decoding pass.
* Future improvements might involve improving opj_decompress, and the image writing logic,
  to use this strategy.