When dynamic caps change in the pipeline leads to a new buffer pool
getting negotiated, the change is not propagated to Qt6GLWindow, which
keeps using the old, now defunct, pool.
Unset current pool on Qt6GLWindow in decide_allocation(). This will
trigger a switch to the new pool inside create().
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8676>
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