* consumed some or all of its input buffer. A pointer to the buffer of consumed
* data, its length and a pointer to the own \a filter_callback structure are passed
* to \a input_cb. The input callback is expected to return a negative value on errors.
* consumed some or all of its input buffer. A pointer to the buffer of consumed
* data, its length and a pointer to the own \a filter_callback structure are passed
* to \a input_cb. The input callback is expected to return a negative value on errors.