x11: xcb_aux_get_screen instead of two hand-rolled screen walks

win.c and xim.c each iterated the setup's roots to find the screen
xcb_connect had chosen; xcb-util, already linked through imdkit, has
xcb_aux_get_screen for that.

win.c

@@ -1,4 +1,5 @@
 #include <xcb/xcb.h>
+#include <xcb/xcb_aux.h>
 #include <xcb/randr.h>
 #include "dat.h"
 #include "fn.h"
@@ -14,17 +15,6 @@ static xcb_atom_t workarea;
 static int hasrandr, imgh, imgw;
 static int shown, ptrx, ptry;	/* the popup, and where the pointer was as it came up */
 
-static xcb_screen_t*
-getscr(xcb_connection_t *c, int n)
-{
-	xcb_screen_iterator_t i;
-
-	for(i = xcb_setup_roots_iterator(xcb_get_setup(c)); i.rem; xcb_screen_next(&i))
-		if(n-- == 0)
-			return i.data;
-	return nil;
-}
-
 static xcb_visualtype_t*
 getvisual(xcb_screen_t *s)
 {
@@ -218,7 +208,7 @@ wininit(void)
 		wincleanup();
 		return 0;
 	}
-	scr = getscr(conn, n);
+	scr = xcb_aux_get_screen(conn, n);
 	if(scr == nil || !validformat(conn, scr)){
 		fprint(2, "strans: popup disabled: unsupported X root format\n");
 		wincleanup();