ImageReader : Optimize isComplete()
ImageReader : Optimize isComplete()

By reading the final tile or scanline, we can determine completeness without reading the whole image.

See http://lists.openimageio.org/pipermail/oiio-dev-openimageio.org/2017-June/000959.html for more details on the approach.

appleseedhq/cortex,appleseedhq/cortex,appleseedhq/cortex