Add gdk_x11_display_get_parent_relative_pattern().

In #1280 (closed) @otte wrote:

There's two ways I think we can go about it:

1. always set ParentRelative when gdk_rgba_is_clear(), the visual supports no alpha and the depths match.

2. Have a new magic API cairo_pattern_t *gdk_x11_display_get_parent_relative_pattern() that returns a cairo pattern per-display singleton that is used by the background setting code to force ParentRelative to be set.

I don't like either solution very much because they have interesting repercussions, but I think they solve the problem and they're both better than having a magic GdkWindow flag.

This is my attempt at 2.