xwayland: Commit after acknowledging configure

Kenny Levinsen requested to merge kennylevinsen/xserver:commit-after-ack into master

If the initial configure event contains non-zero dimensions, we would end up committing a buffer before libdecor ack'ed the configure which is a protocol error. Moving the attach/commit out of xwl_output update logic to the end of the configure handler fixes it, even if slightly awkward.

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