We did several things to enable the new memory logic in msdkdec:
(1) We always use video memory for decoder in linux path;
(2) We give negotiated pool to alloc_pool stored in GstMsdkContext which
will be used in callback mfxFrameAllocator:Alloc to alloc surfaces as
MediaSDK needs, and this pool is also available for decoder itself;
(3) We modify decide_allocation process, that is we make pool negotiaion
before gst_msdk_init_decoder to ensure the pool is decided and ready for
use in mfxFrameAllocator:Alloc callback; then we will consider the case
when we need to do the gpu to cpu copy.
(4) In gst_msdkdec_finish_task, we modify the way for copy following the
logic in (3).
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3439>
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