NS (darwin): What is the utility of update_window?

[Keith David Bershatsky]

PS:  I am aware that the desired_matrix is compared to the current_matrix, and 
that the current_matrix is brought up to date during update_window....  The 
question in the initial post was essentially geared towards ascertaining 
whether anything is actually changed on the glass during update_window on the 
NS port of Emacs.