present: recursively set window pixmaps on flip

present: recursively set window pixmaps on flip

Newly created windows inherit the pixmap of their parent, similarly,
reparenting a tree inherits the pixmap of the destination tree.

Making present preserve the invariant that unredirected windows always
have the same pixmap as their parent ensures that the above cases work
correctly.

v2: name the recursive function to 'set_tree_pixmap' instead of 'set_window_pixmap'

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