[xwl] Fix xcb to wayland clipboard
A seemingly unrelated change to cursors broke xwayland's clipboard syncing, presumably because of a change in how startup is ordered with regards to syncing between our internal wayland connection causing a crash. It was fixed with a flush, but ultimately guarding things coming up in the natural order works better. Long term plans are to finish removing the internal connection. See xwayland_selections_test in autotests.
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