avcodec/roqvideo: Use dedicated context for encoder

avcodec/roqvideo: Use dedicated context for encoder

Up until now, the RoQ video decoder and encoder used the same context;
and said context contained several fields that are only used by the
encoder. This commit changes this and uses a dedicated context for the
encoder; it contains the common context as first element in order to use
functions common to the de- and encoder.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>