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.

xim.c

@@ -5,6 +5,7 @@
 #include <locale.h>
 #include <poll.h>
 #include <xcb/xcb.h>
+#include <xcb/xcb_aux.h>
 #include <xcb/xkb.h>
 #include <xkbcommon/xkbcommon.h>
 #include <xkbcommon/xkbcommon-compose.h>
@@ -132,18 +133,6 @@ kwatch(void)
 		events, parts, parts, nil);
 }
 
-static xcb_screen_t*
-getscreen(int scr)
-{
-	xcb_screen_iterator_t iter;
-
-	iter = xcb_setup_roots_iterator(xcb_get_setup(conn));
-	for(; iter.rem; scr--, xcb_screen_next(&iter))
-		if(scr == 0)
-			return iter.data;
-	return nil;
-}
-
 static int
 translate(xcb_window_t win, int x, int y, Caret *caret)
 {
@@ -550,7 +539,7 @@ ximinit(void)
 		ximlog("cannot connect to X server");
 		return -1;
 	}
-	screen = getscreen(scr);
+	screen = xcb_aux_get_screen(conn, scr);
 	if(screen == nil){
 		ximlog("cannot find X screen");
 		return -1;