shm patches for 1.20.6

Merged Matt Turner requested to merge mattst88/xserver:for-1.20.6 into server-1.20-branch

Fixes: https://bugreports.qt.io/browse/QTBUG-72535

so seems valuable to have in the stable branch. I've been shipping them in Gentoo since 1.20.4 with no ill effects.

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