xcb: Remove need for QXCBScreen to resolve QXcbGlIntegration

David Edmundsonrequested to merge
(removed):kde/5.15 into kde/5.15

Change-Id: I9de4f47bfdf88c92959f210e05c1fc1e8a459cde Reviewed-by: Liang Qi liang.qi@qt.io (cherry picked from commit 590ac717)

This patch is slightly modified to still compile: m_connection -> defaultConnection() but will behave the same.

Fixes: https://bugs.kde.org/show_bug.cgi?id=414805

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