decoder: get rid of GstAdapter-based infrastructure
Submitted by Gwenole Beauchesne
The use of GstAdapter in libgstvaapi is not very convenient without explicit specification of the source buffers granularity (frame, packet, random). More specifically, looking for start/sync codes could be more efficient if simple list of buffers was maintained. i.e. fast scan_for_start_codes() + minor additional care needed at boundaries.