Move glx version check functionality to platform plugin

Summary:
Glx version is only required once in the standalone x11 platform. No
need to have that in the shared kwinglutils library.

Removes one xlib usage from shared kwin.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3329