wined3d: Track if a context's private hdc has had its pixel format set, so we don't need to check it.

This currently only applies to the swapchain backup DC, but it will soon be
used for surfaces created using the WGL_WINE_surface extension.  Also, there
are a couple of cases where ddraw creates private windows and passes them in.
It could be extended to those.

wined3d: When restoring pixel format in context_release(), mark the context as needing to be set on the next context_acquire().

If we had to restore the pixel format then we know it won't be what we need
when the app calls into wined3d again.  This helps avoid the GetPixelFormat()
in context_acquire().

If context_set_gl_context() fails, it marks the context as invalid.  It still
has an HDC, though, and context_update_window() would lose the reference to
it.