We don't want to set ParentRelative when:
- the parent window is NULL
In that case we are unsure about the depth, so better err on the side
of caution and avoid a BadMatch by accepting ugly output.
- the cairo pattern is in an error status
This should never happen - unless you start up in OOM - but better
be safe than sorry.
Might help with the spurious crashes in #1280.
6a47e9a8b9