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(-)