FBO stretchrect marks SFLAG_INDRAWABLE up to date, which currently
also marks SFLAG_INTEXTURE up to date. However, this will change when
we enable rendering to the srgb copy, in which case the drawable could
be equal to the srgb copy, not the rgb copy.

The D3D context is potentially destroyed, so we need to clear the thread's
current D3D context. Found by Rico Schüller.

This is just EXT_provoking_vertex promoted to ARB.

This fixes a compile breakage with older gcc versions.

wined3d: Don't call wglMakeCurrent(NULL, NULL) in context_set_current() if the current context is NULL.