Fix selection bug

Fix selection bug
After the commit named "Remove timeout in the main loop", selection is not
working in the proper way. After selecting something, press mouse button in
a line outside of selection causes an incorrect highlight. This patch fix
the problem forcing a draw after the press event, but this is only a fast
hack. Real solution means rewriting selection code.
---
 st.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)