present/wnmd: Allow flipping if the window pixmap matches the toplevel's

Instead of testing window->redirectDraw.

With Xwayland, the toplevel window is always redirected, so this would
unnecessarily preclude flipping there in some cases, e.g. with wlroots
based Wayland compositors or with fullscreen X11 windows in weston.

Fixes issue #631.
(Cherry picked from commit a093a885)