That way we ensure that the GL context(s) get disposed, which they
previously weren't due to them still being the current context.
This also implicitly adds testing of gLContext destruction, which
previously wasn't ever done by any test.
This happens in the real world when using the inspector to look at a
node recording of a GStreamer video while the video is still playing.
GStreamer will use the GL context in a different thread while we are
busy trying to download it.
A test is included.
