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)
Status Job ID Name Coverage
  Docker Image
passed debian-testing #225567


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


passed meson-build-and-test #225569