wined3d: Use the client area dimensions instead of the backbuffer dimensions for the Vulkan swapchain.

Often these are the same, and even when not, the minimum/maximum image extents
will often end up forcing the swapchain dimensions to be the same as the
client area. However, some implementations will allow the swapchain to be
created with dimensions different from the client area, but then return
VK_SUBOPTIMAL_KHR from vkQueuePresentKHR().
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>