The libplacebo sync abstraction is deprecated and we should be using
the more explicit Vulkan semaphore helpers instead. This means that
more of the book keeping moves to our side, but it's not too bad.
There are two main things going on here:
1. After a lot of back and forth, I decided to write the new code with
timeline semaphores to streamline things, and that also means all
the variables are separate - which makes the #ifdefs easier to read.
Which is important because:
2. While pl_sync owned the exported fd/handle, pl_vulkan_sem does not,
so we are responsible for managing them. That means reversing the
previous logic - we now can pass an original fd to CUDA and never
think about it again, while we have to clean up a Win32 Handle
because CUDA will not take ownership.
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