nvcodec: Refactor GstCudaMemory abstraction

nvcodec: Refactor GstCudaMemory abstraction

* Hide GstCudaMemory member variables
* Make GstCudaAllocator object GstCudaContext independent
* Set offset/stride of memory correctly via video meta
* Drop GST_BUFFER_POOL_OPTION_VIDEO_ALIGNMENT support.
  This implementation actually does not support custom alignment
  because we allocate device memory via cuMemAllocPitch
  of which alignment is almost uncontrollable

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1834>