(cherry picked from commit e1c5d56b)

Factorises out existing surface initialisation code to reuse.

(cherry picked from commit eaa5e071)

(cherry picked from commit aa51bb3d)

Signed-off-by: Maxym Dmytrychenko <maxym.dmytrychenko@intel.com>

Uploading/downloading data through VPP may not work for some formats, in
that case we can still try to call av_hwframe_transfer_data() on the
child context.

Signed-off-by: Maxym Dmytrychenko <maxym.dmytrychenko@intel.com>

Certain pixel formats (e.g. P8) might not be supported for
download/upload through VPP operations, but can still be used otherwise.

Signed-off-by: Maxym Dmytrychenko <maxym.dmytrychenko@intel.com>

When using GPU surfaces with QSV, one needs to supply a frame allocator,
which will be invoked to pass surface pools to libmfx.
For encoding, this allocator gets invoked not only for the pool of input
frames, but also for a separate pool of (apparently) reconstructed frames
and another pool of MFX_FOURCC_P8, which on Windows needs to return
D3DFMT_P8 D3D surfaces. Those are probably used to store the encoded
bitstream on the GPU.

Signed-off-by: Maxym Dmytrychenko <maxym.dmytrychenko@intel.com>