nine: implement DRI3PresentGroup_CreateAdditionalPresent

The implementation is probably not optimal, there are the following questions:

. what if two swapchain render to the same X drawable (even if different Win HWND)
Current implementation is going to behave very badly there
. spec is not precise here: on which screen should we be on ? Since we don't support
multiple screen/displays yet, this is not yet pb.
. What about focus window ?

Signed-off-by: Axel Davy <axel.davy@ens.fr>