Use the z parameter to specify slice height

before the --z option in tofu didn't have any effect

src/ufo-anka-backproject-task.c

@@ -350,7 +350,7 @@ ufo_anka_backproject_task_process (UfoTask *task,
         z_region[1] = EXTRACT_FLOAT (priv->region, 2);
         z_ends[1] = EXTRACT_FLOAT (priv->region, 1);
     } else {
-        z_ends[0] = priv->z;
+        z_ends[0] = z_region[0] = priv->z;
         z_ends[1] = priv->z + 1.0f;
     }
     if (priv->parameter == PARAM_CENTER) {