made Xinerama screen index customizable

dwm.c

@@ -1554,10 +1554,10 @@ updategeom(void) {
 	/* window area geometry */
 	if(XineramaIsActive(dpy)) {
 		info = XineramaQueryScreens(dpy, &i);
-		wx = info[0].x_org;
-		wy = showbar && topbar ? info[0].y_org + bh : info[0].y_org;
-		ww = info[0].width;
-		wh = showbar ? info[0].height - bh : info[0].height;
+		wx = info[xidx].x_org;
+		wy = showbar && topbar ?  info[xidx].y_org + bh : info[xidx].y_org;
+		ww = info[xidx].width;
+		wh = showbar ? info[xidx].height - bh : info[xidx].height;
 		XFree(info);
 	}
 	else