avcodec/encode: restructure the core encoding code

avcodec/encode: restructure the core encoding code

This commit follows the same logic as 061a0c14bb, but for the encode API: The
new public encoding API will no longer be a wrapper around the old deprecated
one, and the internal API used by the encoders now consists of a single
receive_packet() callback that pulls frames as required.

amf encoders adapted by James Almer
librav1e encoder adapted by James Almer
nvidia encoders adapted by James Almer
MediaFoundation encoders adapted by James Almer
vaapi encoders adapted by Linjie Fu
v4l2_m2m encoders adapted by Andriy Gelman

Signed-off-by: James Almer <jamrial@gmail.com>