WGL_SWAP_METHOD attribute is not handled. Explicitly skip it in ConvertAttribWGLtoGLX and always return WGL_SWAP_UNDEFINED_ARB in X11DRV_wglGetPixelFormatAttribivARB, because buffers are always in undefined state after swap in GLX.
There's GLX_OML_swap_method. It is supported on fglrx, and I think it is supported on the open source drivers as well