%{S}: don't clear other outputs

When asking lemonbar to display text to a given ouput with %{S}, don't clear
all outputs first in order to render the text.  The output will persist on
the other outputs until other text is rendered there.

lemonbar.c

@@ -666,6 +666,7 @@ parse (char *text)
 
                               p++;
                               pos_x = 0;
+                              fill_rect(cur_mon->pixmap, gc[GC_CLEAR], 0, 0, cur_mon->width, bh);
                               break;
                     case 'O':
                               errno = 0;