wined3d: Control the onscreen depth stencil format in the swapchain.

wined3d: Control the onscreen depth stencil format in the swapchain.

This allows the swapchain to know what depth format its window contexts have to see if the
requested depth format is compatible or a FBO fallback is needed, and it will be needed to
set the onscreen format to the requested auto depth stencil format instead of the
let's-hope-it-fits D24_UNORM_S8_UINT format.