ScreenBuf::Flush() optimization added in 2.0.1077 sometimes led to slow screen redraw

git-svn-id: http://farmanager.googlecode.com/svn/trunk@11515 ea59307d-ff68-5b67-5a21-acd59c66a581
some more redraw optimizations

git-svn-id: http://farmanager.googlecode.com/svn/trunk@11517 ea59307d-ff68-5b67-5a21-acd59c66a581