Check alternative screen before drawing box selection

Check alternative screen before drawing box selection

Some programs use the alternative screen (vi, less, ...), whose
content is different of the main screen. If you select text in one of
the screen, you don't wait the box selection is painted in the other
screen, so it is necessary check if the selection was done in the same
screen we are going to paint. Before to this commit, you could do
something like:

	$ LESS="" ls | less
	(select some code)
	q

and selection box remains drawing in the main screen, but the content
of selection keeps text of the alternate screen.
---
 st.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)