avcodec/mpegpicture: Always reset mbskip_table

Codecs call ff_find_unused_picture() to get the index of
an unused picture; said picture may have buffers left
from using it previously (these buffers are intentionally
not unreferenced so that it might be possible to reuse them;
they are only reused when they are writable, otherwise
they are replaced by new, zeroed buffers). They should
not make any assumptions about which picture they get.

Yet this is not true for mbskip_table and damaged bitstreams.
When one returns old unused slots randomly, the output
becomes nondeterministic. This can't happen now (see below),
but it will be possible once mpegpicture uses proper pools
for the picture tables.

The following discussion uses the sample created via
ffmpeg -bitexact -i fate-suite/svq3/Vertical400kbit.sorenson3.mov -ps 50 -bf 2 -bitexact -an -qscale 5 -ss 40 -error_rate 4 -threads 1 out.avi

When decoding this with one thread, the slots are as follows:
Cur 0 (type I), last -1, Next -1; cur refcount -1, not reusing buffers
Cur 1 (type P), last -1, Next 0; cur refcount -1, not reusing buffers
Cur 2 (type B), last 0, Next 1; cur refcount -1, not reusing buffers
Cur 2 (type B), last 0, Next 1; cur refcount 2, not reusing buffers
Cur 0 (type P), last 0, Next 1; cur refcount 2, not reusing buffers
Cur 2 (type B), last 1, Next 0; cur refcount 1, reusing buffers
Cur 2 (type B), last 1, Next 0; cur refcount 2, not reusing buffers
Cur 1 (type P), last 1, Next 0; cur refcount 2, not reusing buffers
Cur 2 (type B), last 0, Next 1; cur refcount 1, reusing buffers
Cur 2 (type B), last 0, Next 1; cur refcount 2, not reusing buffers
Cur 0 (type I), last 0, Next 1; cur refcount 2, not reusing buffers
Cur 2 (type B), last 1, Next 0; cur refcount 1, reusing buffers
Cur 2 (type B), last 1, Next 0; cur refcount 2, not reusing buffers
Cur 1 (type P), last 1, Next 0; cur refcount 2, not reusing buffers

After the slots have been filled initially, the buffers are only
reused for the first B-frame in a B-frame chain:
a) When the new picture is an I or a P frame, the slot of the backward
reference is cleared and reused for the new frame (as has been said,
"cleared" does not mean that the auxiliary buffers have been
unreferenced). Given that not only the slot in the picture array,
but also MpegEncContext.last_picture contain references to these
auxiliary buffers, they are not writable and are therefore not reused,
but replaced by new, zero-allocated buffers.
b) When the new picture is the first B-frame in a B-frame chain,
the two reference slots are kept as-is and one gets a slot that
does not share its auxiliary buffers with any of MpegEncContext.
current_picture, last_picture, next_picture. The buffers are
therefore writable and are reused.
c) When the new picture is a B-frame that is not the first frame
in a B-frame chain, ff_mpv_frame_start() reuses the slot occupied
by the preceding B-frame. Said slot shares its auxilary buffers
with MpegEncContext.current_picture, so that they are not considered
writable and are therefore not reused.

When using frame-threading, the slots are made to match the one
from the last thread, so that the above analysis is mostly the same
with one exception: Other threads may also have references to these
buffers, so that initial B-frames of a B-frame chain need no longer
have writable/reusable buffers. In particular, all I and P-frames
always use new, zeroed buffers. Because only the mbskip_tables of
I- and P-frames are ever used, it follows that there is currently
no problem with using stale values for them at all.

Yet as the analysis shows this is very fragile:
1. MpegEncContext.(current|last|next)_picture need not have
references of their own, but they have them and this influences
the writability decision.
2. It would not work if the slots were returned in a truely random
fashion or if there were a proper pool used.

Therefore this commit always resets said buffer. This is in preparation
for actually adding such a pool (where the checksums for said sample
would otherwise be depending on the number of threads used for
decoding).

Future commits will restrict this to only the codecs for which
it is necessary (namely the MPEG-4 decoder).

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

libavcodec/mpegpicture.c

@@ -238,6 +238,7 @@ int ff_alloc_picture(AVCodecContext *avctx, Picture *pic, MotionEstContext *me,
         goto fail;
 
     pic->mbskip_table = pic->mbskip_table_buf->data;
+    memset(pic->mbskip_table, 0, pic->mbskip_table_buf->size);
     pic->qscale_table = pic->qscale_table_buf->data + 2 * mb_stride + 1;
     pic->mb_type      = (uint32_t*)pic->mb_type_buf->data + 2 * mb_stride + 1;