avcodec: add a get_encode_buffer() callback to AVCodecContext

This callback is functionally the same as get_buffer2() is for decoders, and
implements for the new encode API the functionality of the old encode API had
where the user could provide their own buffers.

Reviewed-by: Lynne <dev@lynne.ee>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: James Almer <jamrial@gmail.com>