decoder: h264: fix closure of "other-field" gap.

decoder: h264: fix closure of "other-field" gap.

When a dummy "other-field" is inserted, it is assumed to inherit the
reference flags from the first field, and the sliding window decoded
reference picture marking process is also executed so that corrupted
frames are moved out as early as possible.

While doing so, we also try to output frames that now contain a single
valid field picture, prior to inserting any other picture into the DPB.
Note: this may be superfluous currently based on the fact that dpb_add()
combines the two most recent pairable fields, but this process would be
further simplified later on.