avcodec/vc1dec: Split VC-1 decoders from msmpeg4

The only msmpeg4 code that is ever executed by the VC-1 based
decoders is ff_msmpeg4_decode_init() and what is directly
reachable from it. This is:
a) A call to av_image_check_size(), then ff_h263_decode_init(),
b) followed by setting [yc]_dc_scale_table and initializing
scantable/permutations.
c) Afterwards, some static tables are initialized.
d) Finally, slice_height is set.

The replacement for ff_msmpeg4_decode_init() performs a)
just like now; it also sets [yc]_dc_scale_table,
but it only initializes inter_scantable and intra_scantable
and not permutated_intra_[hv]_scantable: The latter are only
used inside decode_mb callbacks which are only called
in ff_h263_decode_frame() which is unused for VC-1.*

The static tables initialized in c) are not used at all by
VC-1 (the ones that are used have been factored out in
previous commits); this avoids touching 327KiB of .bss.

slice_height is also not used by the VC-1 decoder (setting
it in ff_msmpeg4_decode_init() is probably redundant after
b34397b4cd780b5692548e7d021ec884c7217dba).

*: It follows from this that the VC-1 decoder is not really
based upon the H.263 decoder either; changing this will
be done in a future commit.

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

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