decoder: introduce lists of units to decode before/after frame.

decoder: introduce lists of units to decode before/after frame.

Theory of operations: all units marked as "slice" are moved to the "units"
list. Since this list only contains slice data units, the prev_slice pointer
was removed. Besides, we now maintain two extra lists of units to be decoded
before or after slice data units.

In particular, all units in the "pre_units" list will be decoded before
GstVaapiDecoder::start_frame() is called and units in the "post_units"
list will be decoded after GstVaapiDecoder::end_frame() is called.