wined3d: Don't overwrite the offscreen depth/stencil surface with undefined data in surface_load_ds_location().

The plan is to get rid of IXMLDOMNode interface in xmlnode object and use 
proper inheritance.

The basic rule is that you can't call anything that takes the user32 / gdi32
lock while under the GL (winex11) lock. As a consequence, you can't call
anything like context_acquire() or context_destroy() either.