DCs can go over int16_t, and when it does, dc_add wraps, so the wrong sign
is reported, which causes errors when decoding more DCs.
Do the prediction in int and narrow to int16 only at store time. The
read_ac_vals() sign change is cosmetic (the value is only ever 0/-1, and
the old code already widened it).
(cherry picked from commit 5784a775df)
2a31f05fca