avcodec/sanm: bl16: fix artifacts in larger videos

The DOS/Windows decoder precomputes a table of linear offsets of
all motion vectors given the current image width.
For larger widths (>=762), the pairs starting at indices 1 and 254
of motion_vectors[] will overflow the int16_t, changing the sign.

Playing back the 800x600 "jonesopn_8.snm" video of "Indiana Jones and
the Infernal Machine" reveals a lot of artifacts and a lot of
"Ignoring invalid motion vector (149, -41)->(136, 0), block size = 8"
messages, hinting at the wrong direction of the motion vectors.

Fix this by doing the calculation that the DOS/Windows players do,
let the value overflow and reextract the "new" mvec x/y components.

0 files changed, 0 insertions, 0 deletions