softwrap: use smooth scrolling when softwrapping is toggled on

If the cursor is near the bottom row when softwrap gets switched on, the
screen should scroll just the amount needed to keep the cursor onscreen.

This fixes https://savannah.gnu.org/bugs/?56317.

Bug existed since softwrapping was introduced in version 2.1.11.

src/nano.c

@@ -1360,7 +1360,9 @@ void do_toggle(int flag)
 			signal_init();
 			break;
 		case SOFTWRAP:
-			if (!ISSET(SOFTWRAP))
+			if (ISSET(SOFTWRAP))
+				edit_redraw(openfile->current, FLOWING);
+			else
 				openfile->firstcolumn = 0;
 			refresh_needed = TRUE;
 			break;