Get a frame's AV_FRAME_DATA_DOWNMIX_INFO side data for editing.
If the side data is absent, it is created and added to the frame.
@param frame the frame for which the side data is to be obtained or created
@return the AVDownmixInfo structure to be edited by the caller, or NULL if the structure cannot be allocated.
See Implementation
Get a frame's AV_FRAME_DATA_DOWNMIX_INFO side data for editing.
If the side data is absent, it is created and added to the frame.
@param frame the frame for which the side data is to be obtained or created
@return the AVDownmixInfo structure to be edited by the caller, or NULL if the structure cannot be allocated.