lavu/hwcontext: Add support for HW -> HW transfers

We are beginning to consider scenarios where a given HW Context
may be able to transfer frames to another HW Context without
passing via system memory - this would usually be when two
contexts represent different APIs on the same device (eg: Vulkan
and CUDA).

This is modelled as a transfer, as we have today, but where both
the src and the dst are hardware frames with hw contexts. We need
to be careful to ensure the contexts are compatible - particularly,
we cannot do transfers where one of the frames has been mapped via
a derived frames context - we can only do transfers for frames that
were directly allocated by the specified context.

Additionally, as we have two hardware contexts, the transfer function
could be implemented by either (or indeed both). To handle this
uncertainty, we explicitly look for ENOSYS as an indicator to try
the transfer in the other direction before giving up.

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