This mirrors a similar change in mpv, which helped reduce vsync jitter
measurements by including the time of frame acquisition in the
`swap_buffers` call. In general, doing things this way around is nicer
for the user. In typical swapchain implementations, it also "does the
right thing" w.r.t actually blocking until the buffer swap.

As a side effect of the necessary internal cache metadata, we also make
the vulkan swapchain more robust against API misuse (especially
out-of-order / non-lockstepped swapchain calls).