wined3d: Ensure all states affected by a change in render_offscreen are marked dirty in ActivateContext.

oleaut32: Clear the input pvar structure in VARIANT_UserUnmarshal to free the data instead of just initializing.

Rework storage to make all its interactions with the underlying data go 
though ReadAt and WriteAt to allow for an ILockBytes underlying layer.
Implement ReadAt and WriteAt in stg_bigblockfile.c for blockfiles.