wined3d: Fix context backup window handling.

[Henri Verbeet]

On 25 July 2017 at 22:35, Sebastian Lackner <sebastian at fds-team.de> wrote:
> -    if (dc == context->hdc && !context->hdc_is_private && WindowFromDC(dc) != context->win_handle)
> +    if (dc == context->hdc && !private && WindowFromDC(dc) != context->win_handle)

While that works, I think I'd feel happier about the code if the "dc",
"private" and "format" parameters just went away.

I.e., the reason they still exist is mostly the backup window path in
context_set_gl_context(), and I don't think there's a reason that
particular path shouldn't just set the appropriate (hdc,
hdc_is_private, hdc_has_format) fields in the context instead.