The GIF reader plug-in passes the Y value of the start of the current scan
instead of the Y value of the area that was most recently modified to
the IIOReadUpdateListener.imageUpdate method.

  Instead of passing the value 'updateMinY', which is constant for a whole
scan, it should pass 'destY', which is incremented as the scan proceeds.

  A regression test will be added in test/javax/imageio/plugins/gif/GIFReadUpdate.java.