This example demonstrates the effect of background_pixmap window
attribute settings of ParentRelative and None, and also of the 
win_gravity window attribute.

The main window has background_pixmap ParentRelative, and the child
has background_pixmap None.  Close observation will tell you the unique
characteristics of each of these settings.

The child window has win_gravity set to NorthEastGravity, so that
the server automatically moves it back into the corner when the main 
window is resized.
