basetransform: set_passthrough() not doing what the doc says

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#792645)

Description

The doc says:

• Always %TRUE for filters which don't implement either a transform
• or transform_ip method.

While the implementation is no-op in the case we pass FALSE and that conditions is met:

if (!passthrough) {
if (bclass->transform_ip || bclass->transform)
trans->priv->passthrough = FALSE;
} else {
trans->priv->passthrough = TRUE;
}

It's also a weird condition to check for filters that only wants to implement submit_input_buffer() and generate_output(), as they need to place a useless stub for one of the transform function in order to make it behave properly.