[lfdevs]

Variant of PR #2 for Ubuntu 26.04.

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• present: disarm the wait-fence callback before re-execution

  A fence notification is one-shot, but re_execute() can leave the Present request alive while it waits for its target MSC. Clear the callback before re-entering the execution path. If that path must wait again, it installs a new callback itself.

  This is required when the Sync trigger walker restarts after a callback: otherwise a still-live request can be called repeatedly in the same scan. It also prevents a later trigger of a reused fence from executing the old request again.

  The same callback-lifetime correction is used by Termux:X11: KiralyCraft/termux-x11@fc534e5

  (cherry picked from commit 3578597)

• xwayland: do not reacquire a copied Present pixmap

  After a COPY Present, Idle permits the client to reuse the source pixmap and reset its wait fence for a later frame. The old request may still be queued for its delayed Complete notification. Re-executing that request must not wait for the source again, because that now belongs to a different presentation.

  Waiting a second time can overwrite blocking_event with the old copied request. That request subsequently completes without draining blocked_queue, leaving newer requests blocked indefinitely.

  Only perform acquisition before the copy. The first wait, asynchronous render fences, and delayed completion scheduling are unchanged.

  (cherry picked from commit 99f0115)