xwayland/rootless: Always paint background for toplevel windows

If a toplevel window has no background, temporarily use the root
window's background for it in PaintWindow.

This avoids presenting uninitialized window contents, which should fix
issue #636.
3 jobs for xwl_paint_window in 8 minutes and 8 seconds (queued for 9 seconds)
latest
Status Job ID Name Coverage
  Docker Image
passed #225567
debian-testing

00:00:34

 
  Build And Test
passed #225568
autotools-build-and-test

00:07:33

passed #225569
meson-build-and-test

00:02:10