Add gdk_x11_display_get_parent_relative_pattern().

Fixes #1280, tray icons not drawing background. This is a magic pattern only
usable for gdk_window_set_background_pattern() that sets the underlying
X window's background to ParentRelative.
2 jobs for GtkTrayIcon_background in 8 minutes and 25 seconds (queued for 4 seconds)
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passed #129208
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00:08:25

manual #129209
win32 allowed to fail manual
msys2-mingw32