This saves one malloc + memcpy per packet
The CodedBitstreamFragment buffer is padded to follow the requirements
of AVPacket.
Reviewed-by: jkqxz
Signed-off-by: James Almer <jamrial@gmail.com>
This makes it easier for users of the CBS API to get alloc/free right -
all subelements use the buffer API so that it's clear how to free them.
It also allows eliding some redundant copies: the packet -> fragment copy
disappears after this change if the input packet is refcounted, and more
codec-specific cases are now possible (but not included in this patch).