Replace homebrew extension matcher with Qt Builtin

Summary:
The functionality of QGLExtensionMatcher has been subsumed in the method
QOpenGLContext::hasExtension(). This replaces several calls to
glGetProcAddress with the Qt versions ctx->getProcAddress, which is able
to catch when these calls are made without an active context.

Reviewers: dkazakov, rempt

Maniphest Tasks: T360

Differential Revision: https://phabricator.kde.org/D233