sdl2: set window ID

This uses the console API to record the window ID of the SDL2 windows.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-id: 20161221003806.22412-4-samuel.thibault@ens-lyon.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>