Summary:
The EglOnXBackend is no longer needed in the core. It's only needed by
the two x11 platform plugins. To best share it, it's moved into a common
directory and compiled into a static library which in turn is linked by
the two plugins.
Reviewers: #plasma
Subscribers: plasma-devel
Projects: #plasma
Differential Revision: https://phabricator.kde.org/D1413
