move i3 sync code into sync_respond (for following commits)

src/sync.c

@@ -0,0 +1,28 @@
+/*
+ * vim:ts=4:sw=4:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ * © 2009 Michael Stapelberg and contributors (see also: LICENSE)
+ *
+ * sync.c: i3 sync protocol: https://i3wm.org/docs/testsuite.html#i3_sync
+ *
+ */
+#include "all.h"
+
+void sync_respond(xcb_window_t window, uint32_t rnd) {
+    DLOG("[i3 sync protocol] Sending random value %d back to X11 window 0x%08x\n", rnd, window);
+
+    void *reply = scalloc(32, 1);
+    xcb_client_message_event_t *ev = reply;
+
+    ev->response_type = XCB_CLIENT_MESSAGE;
+    ev->window = window;
+    ev->type = A_I3_SYNC;
+    ev->format = 32;
+    ev->data.data32[0] = window;
+    ev->data.data32[1] = rnd;
+
+    xcb_send_event(conn, false, window, XCB_EVENT_MASK_NO_EVENT, (char *)ev);
+    xcb_flush(conn);
+    free(reply);
+}