vo_gpu_next: guard from cache save conflict

If multiple instances of mpv are closed at the same time, they will
write to the same temporary file. Fix that by using unique temporary
file.

video/out/vo_gpu_next.c

@@ -1557,10 +1557,15 @@ static void save_cache_files(struct priv *p)
 
         if (!target_file)
             continue;
-        char *tmp = talloc_asprintf(ta_ctx, "%s~", target_file);
-        FILE *cache = fopen(tmp, "wb");
-        if (!cache)
+        char *tmp = talloc_asprintf(ta_ctx, "%sXXXXXX", target_file);
+        int fd = mkstemp(tmp);
+        if (fd < 0)
             continue;
+        FILE *cache = fdopen(fd, "wb");
+        if (!cache) {
+            close(fd);
+            continue;
+        }
         int ret = pl_cache_save_file(target_cache, cache);
         if (same_cache)
             ret += pl_cache_save_file(p->icc_cache, cache);