vaapiencode: fix support for raw YUV sink buffers.

vaapiencode: fix support for raw YUV sink buffers.

Allow vaapiencode plug-in elements to encode from raw YUV buffers.
The most efficient way to do so is to let the vaapiencode elements
allocate a buffer pool, and subsequently buffers from it. This means
that upstream elements are expected to honour downstream pools.

If upstream elements insist on providing their own allocated buffers
to the vaapiencode elements, then it possibly would be more efficient
to insert a vaapipostproc element before the vaapiencode element.
This is because vaapipostproc currently has better support than other
elements for "foreign" raw YUV buffers.