[st][PATCH] xclearwin clears the window

When an OCS sequence was used to change the bg color, the borders where
dirty. This simple patch just clears the window before the redraw of the
terminal when the bg color has been changed. This is apparently enough
and seams to be very smooth. There was a TODO comment for it on the st.c
file, which I removed.

st.c

@@ -2021,10 +2021,8 @@ strhandle(void)
 				fprintf(stderr, "erresc: invalid color j=%d, p=%s\n",
 				        j, p ? p : "(null)");
 			} else {
-				/*
-				 * TODO if defaultbg color is changed, borders
-				 * are dirty
-				 */
+				if (j == defaultbg)
+					xclearwin();
 				redraw();
 			}
 			return;

win.h

@@ -40,3 +40,4 @@ void xsetpointermotion(int);
 void xsetsel(char *);
 int xstartdraw(void);
 void xximspot(int, int);
+void xclearwin(void);