Prevent endless loop in checkGLError due to GL_CONTEXT_LOST

Summary:
The GL_CONTEXT_LOST flag is not reset when calling glGetError. This of
course bites with: "Thus, glGetError should always be called in a loop,
until it returns GL_NO_ERROR, if all error flags are to be reset."
(see https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/glGetError.xhtml)

As KWin calls checkGLError from init call it can result in a freeze of
KWin. This is a regression reported multiple times since the release of
5.10.3 which enabled GLX_NV_robustness_video_memory_purge extension.

Please note that I am not able to test this change. I do not have an
NVIDIA card and are not hiting the problem. I have no way to simulate
the code. I do not know whether the change will fix the problem, it is
based on what others do. Inspiration for this change is mostly from
mutter: https://git.gnome.org/browse/mutter/commit/?id=d4d2bf0f6c1737256b921c4f1dedd3a95138cab9

BUG: 381870
FIXED-IN: 5.10.3.1

Test Plan: See above, I can only compile check the change

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6464