[platforms/x11] Add support for GLX_NV_robustness_video_memory_purge

Summary:
Based on earlier phab-request D2079. Adds the nvidia memory purge
extension to the robust context creation.

BUG: 344326
FIXED-IN: 5.10.3

Test Plan: Only compile tested as I don't have an NVIDIA card.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D6344