pietty with gnu screen scroll buffer

Enable mouse scroll with the GNU screen scrollback:

1. Check Option > More Options > Terminal > Features > Disable switching to alternate terminal screen
2. Make sure the PieTTY scrollback matches the setting of GNU screen. If not, change it at Option > More Options > Window > Lines of scrollback
3. Add "termcapinfo xterm*|xs|rxvt|terminal ti@:te@" to ~/.screenrc