Recent versions of libplacebo have required Vulkan versions incompatible
with lavu Vulkan hwcontexts. While this is expected to change
eventually, breaking vf_libplacebo every time there is such a transition
period is obviously undesired behavior, as the following sea of bug
reports shows.

This commit adds a fallback path for init_vulkan failures which simply
creates an internal device if there was no user-supplied Vulkan hwaccel.
Useful when no interop with lavu vulkan hwframes is needed or desired,
and makes using this filter easier inside certain applications.

Fixes: https://github.com/haasn/libplacebo/issues/170
Fixes: https://github.com/mpv-player/mpv/issues/9589#issuecomment-1535432185
Fixes: https://github.com/mpv-player/mpv/issues/11363
Fixes: https://github.com/mpv-player/mpv/issues/11685#issuecomment-1546627082
Closes: https://code.videolan.org/videolan/libplacebo/-/issues/270