[dev] [st] Scroll selection patch

Hi,

One thing from st that bugs me a little: whenever I select some text
with the mouse it remains marked even if I type in the same line and
change its content. The same happens if I keep typing and the window
scrolls, since the selection box does not scroll along with it. In
either case, a new piece of text seems to be selected, but this "new
selection" is of course not copied to the clipboard, which feels
annoying.

This patch solves this problem with a behavior akin to that of
xterm/urxvt: scrolling the selection box with the screen and hiding it
if you type in its line or clear the screen.

Cheers,
Rafa

PS: I tried looking both at xterm and urxvt source for a minute;
almost ripped my eyes off