present: Set window pixmap to flipped pixmap

present: Set window pixmap to flipped pixmap

This makes other drawing to the window appear on the screen.

Note that no child windows can be affected because only full-screen
windows are eligible for flipping, and so we only need to set pixmap
for the window itself.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>