OpenGL Renderer: fix race condition in shareContext

Render::shareContext() accesses m_submissionContext and m_shareContext,
which are initialized in Renderer::initialize(), so make sure
shareContext() and initialize() don't run simultaneously.

Pick-to: 6.12 6.11 6.8
Change-Id: I4fddaa57297facdb4ffff6d550da6d409e7f8ad6
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>