Reference shm pool for destroyed buffers

Currently, if an application is closed, all of its shm buffers will be
destroyed. However, the compositor may want to access them to fade out
the window.

With this change, the underlying shm pool will be referenced when a shm
buffer is destroyed. So, the compositor can access buffer data even if
the buffer is destroyed.
1 job for !263 with work/delayed-commit-client-buffer in 5 minutes and 18 seconds (queued for 5 seconds)
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passed linux kf5-qt5 qt5.15 #119266

00:05:18

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