lavc: use a separate field for exporting audio encoder padding

Currently, the amount of padding inserted at the beginning by some audio
encoders, is exported through AVCodecContext.delay. However
- the term 'delay' is heavily overloaded and can have multiple different
  meanings even in the case of audio encoding.
- this field has entirely different meanings, depending on whether the
  codec context is used for encoding or decoding (and has yet another
  different meaning for video), preventing generic handling of the codec
  context.

Therefore, add a new field -- AVCodecContext.initial_padding. It could
conceivably be used for decoding as well at a later point.
lavf: use initial_padding instead of deprecated delay
movenc: Write tfdt atoms

These allow getting the absolute start timestamp of a fragment
without reading preceding timestamps. This fixes sync between
tracks if starting from fragments in different streams that don't
align exactly.

This also is a prerequisite for producing DASH content.

Signed-off-by: Martin Storsjö <martin@martin.st>
mov: Handle tfdt atoms

Signed-off-by: Martin Storsjö <martin@martin.st>