[9fans] bug in rio

[Mike Haertel <mike@ducky.net>]

to reproduce:

1.  create a window and enable automatic scrolling on the button 2 menu.
call this Window 1.
start a process in that window that will continuously print output, e.g.:

	while (~ x x) {
		echo hi
		sleep 1
	}

2.  move another window (call it Window 2) slowly across the front of
the scrolling window.  the borders of Window 2 will be "captured" by
the block copies due to the scrolling in Window 1.  you will see
garbage copies of the border slowly scroll off the top of Window 1.

i don't know any way to make the garbage go away except by hiding
and then unhiding Window 1.  unlike other window systems i have used,
rio does not seem to have a "refresh" command.  i imagine this is because
reliable backing store ought to make that unnecessary.  alas...

(N.B. the lack of a refresh command is also annoying in the case
that kernel messages pollute your display; i occasionally get stray
interrupt messages that seem to somehow be associated with the Tekram
scsi controller.)