h264: don't clobber mmco opcode tables for non-first slice headers.

Clobbering these tables will temporarily clobber the template used
as a basis for other threads to start decoding from. If the other
decoding thread updates from the template right at that moment,
subsequent threads will get invalid (or, usually, none at all) mmco
tables. This leads to invalid reference lists and subsequent decode
failures.

Therefore, instead, decode the mmco tables only for the first slice in
a field or frame. For other slices, decode the bits and ensure they
are identical to the mmco tables in the first slice, but don't ever
clobber the context state. This prevents other threads from using a
clobbered/invalid template as starting point for decoding, and thus
fixes decoding in these cases.

This fixes occasional (~1%) failures of h264-conformance-mr1_bt_a with
frame-multithreading enabled.

(cherry picked from commit bad446e251405dc250c3cbee199072e083a1e4b9)

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
h264: fix ff_generate_sliding_window_mmcos() prototype.

It's been returning an error value since
bad446e251405dc250c3cbee199072e083a1e4b9

Also check for the errors it returns.
(cherry picked from commit ea382767ad2191acbe97e90624059723e15f0e4b)

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
h264: add 3 pixels below for subpixel filter wait position

If the motion vector is at a subpixel position, we need 3 pixels below
the motion vector's wholepel position available, not 2, since the MC
filter is a sixtap filter for the hpel position, and then a bilin filter
for the qpel position.

This patch fixes highly irreproducible (0.1%) fate failures in frame 2
and 4 of h264-conformance-cama2_vtc_b (e.g. first P-frame, first field,
last line of MB x=40,y=2 and second field and last lines of MBs x=39-40,
y=3). These used pre-loopfilter instead of post-loopfilter data because
the await_progress() waited for one line too little in that field, and
the motion vector of these particular MBs happened to align exactly to a
position where that demonstrates the bug.

CC: libav-stable@libav.org

(cherry picked from commit fb845ffdd335a1efd6dfd43e8adeb530397b348e)

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>