avcodec/mpegvideo: Enable private contexts

MpegEncContext is used by many different codecs and
every one of these uses just a subset of its fields.
If one tries to separate this and e.g. add a real MpegContext
and extension structures (say MpegDecContext and MpegEncContext),
one runs into two difficulties:

a) Some code is shared between decoder and encoder of
the same format and they therefore use the same contexts,
either MpegEncContext itself or identical extensions thereof.
The latter is the case for H.261 as well as WMV2.
b) In case of slice threading, the generic code can only allocate
and initialize the structure it knows about; right now this is
an MpegEncContext. If the codec has an even more extensive structure,
it is only available for the main thread's MpegEncContext.
Fixing this would involve making ff_mpv_common_init() aware
of the size the size of slice context to allocate and would be
part of separating the main thread's context from the slice contexts
in general.

This commit only intends to tackle the first issue by adding
a pointer to MpegEncContext that codecs can set to a common
context so that the aforementioned codecs can use this context
(together with the MpegEncContext) in their common code.
This will allow to move fields only used by the main thread
to more specialized contexts.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

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