Remove window flicker on start by clearing the window with black after creation.

slock.c

@@ -999,6 +999,10 @@ lockscreen(Display *dpy, struct xrandr *rr, int screen)
                               DefaultVisual(dpy, lock->screen),
                               CWOverrideRedirect | CWBackPixel, &wa);
 #endif
+
+    XSetWindowBackground(dpy, lock->win, BlackPixel(dpy, screen));
+    XClearWindow(dpy, lock->win);
+
     lock->pmap = XCreateBitmapFromData(dpy, lock->win, curs, 8, 8);
     invisible = XCreatePixmapCursor(dpy, lock->pmap, lock->pmap,
                                     &color, &color, 0, 0);