For some reason, this was never set, which meant all **raw** AAC in ADTS
streams, except faac, had extra samples at the start.

Despite this being a standard MDCT-based codec with a frame size of 1024,
hence a delay of 1024 samples at the start, all major encoders, excluding
faac and FFmpeg, use 2048 samples of padding.

Because this is also such a mess, make it an option to allow for
users to configure the amount of padding.

The FFmpeg encoder will be modified to also output 2048 samples of padding
at the start, to make it in line with other encoders.

Yes, this leaves FATE pretty sad. Will be fixed by another patch once
the discussions have completed as to what should be the default.