Split decode_buffer() into the core infrastructure that determines
the NAL units contained in the adapter and the actual function that
decodes the NAL unit.
Maintaining the sub-buffer is rather suboptimal especially since we
were also maintaining a GstAdapter. Now, we only use the GstAdapter
thus requiring minor extra parsing when receiving avcC buffers.
Unlike what VA-API documentation defines, the slice_data_bit_offset
represents the offset to the first macroblock in the slice data, minus
any emulation prevention bytes in the slice_header().
This fix copes with binary-only VA drivers that won't be fixed any
time soon. Besides, this aligns with the current FFmpeg behaviour
that was based on those proprietary drivers implementing the API
incorrectly.
This ensures the VA context is clear when the encoded resolution
changes. i.e. make sure older picture is decoded with the older
VA context before it changes.
Only the explicit pred_weight_table(), possibly with the inferred default
values, shall be required. e.g. don't fill in the table if weighted_pred_flag
is not set for P/SP slices.
