The last commit fixes glitches in Horizon Zero Dawn with vsync on, for reasons stated in the comment there.
The wined3d implementation already blocks the client's call to `Present()` when `vkAcquireNextImageKHR()` blocks, so is roughly equivalent to this when frame latency and buffer count are equal. It has a frame latency implementation which unblocks after calling `vkQueuePresentKHR()`, rather than waiting for its execution to complete. I think the d3d12 implementation is supposed to do the latter, though using `KHR_present_wait` may be unnecessary overkill. Waiting on queue execution comes with a performance cost either way, so without a compelling reason for a strict implementation, something similar to wined3d makes sense.
Another potential issue is after the call to `ID3D12CommandQueue_Signal()` in `d3d12_swapchain_present()`, the client is free to enqueue more commands when `Present()` returns, so by the time the signal is executed and the swapchain worker blits the image and calls `vkQueuePresentKHR()`, these commands are not necessarily enqueued immediately after the `Signal()`. This may not always be harmless.
-- v2: dxgi: Wait for completion of a pending read of the next buffer before returning from d3d12_swapchain_Present(). dxgi: Wake the condition variable after leaving the critical section in d3d12_swapchain_resize_buffers(). dxgi: Wake the condition variable after leaving the critical section in d3d12_swapchain_present().