fix bogus fb/drawable information (fixes xdemos/wincopy)

fix bogus fb/drawable information (fixes xdemos/wincopy)

the framebuffer objects attached to drawables can have invalidate state
associated with them, since for the window framebuffer this is per-context
state and not per-fbo state. Since drivers may rely on that information
(otherwise would need to check if currently the window-framebuffer is
bound in a lot of places) fix it up in _mesa_make_current (ugly).