avcodec/dvenc: Avoid copying packet data, allow user-supplied buffers
When the packet size is known in advance like here, one can avoid
an intermediate buffer for the packet data; this also makes it easy
to allow user-supplied buffers. Only one thing needed to be changed:
The earlier code relied on the buffer having been initially zeroed
by av_fast_padded_malloc(), so one now needs to zero the packet at
first.
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>