patch 9.0.0640: cannot scroll by screen line if a line wraps

Problem:    Cannot scroll by screen line if a line wraps.
Solution:   Add the 'smoothscroll' option.  Only works for CTRL-E and CTRL-Y
            so far.
diff --git a/src/option.c b/src/option.c
index 6052b38..6408e24 100644
--- a/src/option.c
+++ b/src/option.c
@@ -2964,6 +2964,15 @@
     }
 #endif
 
+    else if ((int *)varp == &curwin->w_p_sms)
+    {
+	if (!curwin->w_p_sms)
+	{
+	    curwin->w_skipcol = 0;
+	    changed_line_abv_curs();
+	}
+    }
+
     // when 'textmode' is set or reset also change 'fileformat'
     else if ((int *)varp == &curbuf->b_p_tx)
     {
@@ -5436,6 +5445,7 @@
 	case PV_RLC:	return (char_u *)&(curwin->w_p_rlc);
 #endif
 	case PV_SCROLL:	return (char_u *)&(curwin->w_p_scr);
+	case PV_SMS:	return (char_u *)&(curwin->w_p_sms);
 	case PV_WRAP:	return (char_u *)&(curwin->w_p_wrap);
 #ifdef FEAT_LINEBREAK
 	case PV_LBR:	return (char_u *)&(curwin->w_p_lbr);