Fix errors from incorrect QImage memory management. (#546)

This should stop the sporadic test failures caused by referencing memory
which has been freed and used by something else.  The fix is to attach the
numpy array with the allocated memory to the QImage, so that the lifetime
of the array is at least as long as that of the QImage.

pyface/ui/qt4/util/image_helpers.py

@@ -96,5 +96,6 @@ def array_to_QImage(array):
     elif channels == 4:
         image = QImage(data.data, width, height, bytes_per_line,
                        QImage.Format_ARGB32)
+    image._numpy_data = data
     return image