Allocate decoder unit earlier in the main parse() function and don't
delegate this task to derived classes. The ultimate purpose is to get
rid of dynamic allocation of decoder units.
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.
Introduce GstVaapiDecoderUnit which represents a fragment of the source
stream to be decoded. For instance, a decode-unit will be a NAL unit for
H.264 streams, an EBDU for VC-1 streams, and a video packet for MPEG-2
streams.
This is a libgstvaapi internal object.
