qpa: Guard against failing to get a swapchain in platform opengl context

Vlad Zahorodniirequested to merge
work/cherry-pick-1d05f98e into Plasma/6.1

QOpenGLContextPrivate::setCurrentContext() was dropped because the platform opengl context no longer uses QOpenGLFramebufferObject, which needed QOpenGLContext::currentContext().

(cherry picked from commit 1d05f98e)

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