app: gimp_get_temp_file() / gimp_temp_name() should create tmp/ if...

... it doesn't exist.
The tmp/ dir in the config folder should already be created by GIMP, but
just in case it is not there, try and create it, since all code calling
these assumes that it exists.

app/core/gimp.c

@@ -1183,6 +1183,13 @@ gimp_get_temp_file (Gimp        *gimp,
 
   dir = gimp_file_new_for_config_path (GIMP_GEGL_CONFIG (gimp->config)->temp_path,
                                        NULL);
+  if (! g_file_query_exists (dir, NULL))
+    {
+      /* Try to make the temp directory if it doesn't exist.
+       * Ignore any error.
+       */
+      g_file_make_directory_with_parents (dir, NULL, NULL);
+    }
   file = g_file_get_child (dir, basename);
   g_free (basename);
   g_object_unref (dir);