Demuxing only. Muxing is disabled as altref frame handling is not
defined in the spec, and there's no way to know the presence of
such frames during stream initialization.

Based on a patch by Steven Liu.

Fixes ticket #7000
Reviewed-by: Carl Eugen Hoyos <ceffmpeg@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>