vf_libplacebo: switch to newer libplacebo helpers

Support for mapping/unmapping hardware frames has been added into
libplacebo itself, so we can scrap this code in favor of using the new
functions. This has the additional benefit of being forwards-compatible
as support for more complicated frame-related state management is added
to libplacebo (e.g. mapping dolby vision metadata).

It's worth pointing out that, technically, this would also allow
`vf_libplacebo` to accept, practically unmodified, other frame types
(e.g. vaapi or drm), or even software input formats. (Although we still
need a vulkan *device* to be available)

To keep things simple, though, retain the current restriction to vulkan
frames. It's possible we could rethink this in a future commit, but for
now I don't want to introduce any more potentially breaking changes.

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