Fixes a bug valgrind found.

    
This essentially reverts 2f69329a.
We really need the visible rect to be set atomically with the rest of
the window position info.

Also avoid dependency on winternl.h.

widl: Consolidate top-level parameter conformance/variance expression writing into a separate function.

Descend through as many pointer types as necessary to find the
fundamental type and determine whether it needs a top-level parameter
conformance/variance expression to be written.

Add support for writing top-level parameter conformance/variance
expressions for non-encapsulated unions.