The current DXGI back buffer index is independent of the current Vulkan
image index when we have user images. We can use this fact to delay
calling vkAcquireNextImageKHR().

This increases GPU utilization from 84% to 100% in the main menu of
Metro Exouds on Nvidia.
Signed-off-by: Józef Kucia <jkucia@codeweavers.com>
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>