xwayland/present: Redirect surface window as needed for page flips
It's needed when the surface window is a depth 24 descendant of a depth 32 toplevel window.
xwl_source_validate
ensures the toplevel window pixmap has valid
contents when a client reads from it, or when the window hierarchy /
geometry changes. It's never called in the normal fullscreen application
case, so there's no GPU copy overhead with that.