Resolve "ISM1 non-diegetic panning uses wrong buffer"

• Related issues: #1009 (closed)
• Requested reviewers: @norvell, @emerit

Reason why this change is needed

Non-diegetic panning for one object (ISM1) uses the output buffer also as object input instead if the transport channel buffer, therefore accessing the wrong time domain samples of the object in non-20ms or JBM rendering.

Description of the change

Use the correct buffer.

Affected operating points

• BE for 20ms, non-BE for discrete ISM with diegetic panning and 5/10ms rendering (fix makes it BE to 20ms)
• Produced bitstream is BE compared to previous state (no encoder change).

Related to #1009 (closed)