We will need this for d3d10, where both dxgi and d3d10core are making wined3d
calls. Right now d3d8/d3d9 also use this to protect their own data, but
eventually we should push this down into wined3d itself and use something a
bit more fine-grained. There's no good reason that doing e.g. a vertex buffer
upload in some thread should block all of wined3d.