QVideoFrame::toImage / qImageFromVideoFrame not safe to call from worker thread

when built for opengl (as in QT_CONFIG(opengl)), qImageFromVideoFrame cannot be used from worker threads:

• qImageFromVideoFrame calls initializeRHI
• initializeRHI calls QRhiGles2InitParams::newFallbackSurface
• QRhiGles2InitParams::newFallbackSurface docs state that "This function must only be called on the gui/main thread"

when running with the wayland qpa plugin, a warning is emitted:
> Attempting to create QWindow-based QOffscreenSurface outside the gui thread. Expect failures

we should either document/assert or try to lift this requirement