show-trailing-whitespace makes M-x zone look ugly

[Chris Moore]

If I enable show-trailing-whitespace and run M-x zone then the display
looks bad.  In the mode where zone is dropping z's down the screen,
there are horizontal stripes, because the buffer is full of trailing
whitespace.

A quick fix for this would probably be to locally bind
show-trailing-whitespace to nil while zone is running.