The ImageReader and ImageWriter classes contain an instance variable
'abortFlag' that may be concurrently read by a thread performing a read
or write and modified by another thread, say the GUI thread, that wishes
to abort the read or write in progress.

  This technique is not thread safe, and should be fixed by making the
'abortFlag' variable private and adding protected 'abortRequested' and
'clearAbortRequest' methods for use by subclasses.