Compile the virtual backend with older Mesa

Summary:
BUG: 403326
FIXED-IN: 5.15.0

Test Plan: N/A

Reviewers: #kwin, fvogt, davidedmundson

Reviewed By: #kwin, fvogt, davidedmundson

Subscribers: kwin, fvogt

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18353