Introduce new decoding process whereby a GstVideoCodecFrame is created
first. Next, input stream buffers are accumulated into a GstAdapter,
that is then passed to the _parse() function. The GstVaapiDecoder object
accumulates all parsed units and when a complete frame or field is
detected, that GstVideoCodecFrame is passed to the _decode() function.

Ultimately, the caller receives a GstVaapiSurfaceProxy if decoding
process was successful.