[viewer] Importing viewer module on macos immediately kills maplotlib windows

On MacOS, importing the viewer module from `dm_control` immediately kills the `matplotlib` plot figures even when it should be ideally blocked with plt.show(). To reproduce the issue run the code below as a script:

```python
import matplotlib
import matplotlib.pyplot as plt

from dm_control import viewer

if __name__ == '__main__':
print(f"Matplotlib backed {matplotlib.get_backend()} ")
plt.plot()
plt.show(block=True)
```

**NOTE** : To reproduce the bug from the above script make sure that backend used by matplotlib (printed on the terminal) is `MacOSX`. When using other non default backends on MacOS the issue goes away.

Looking into it I found that the issue comes from https://github.com/deepmind/dm_control/blob/3c67a42d021b97808fcc075c0c468d78f0c2233f/dm_control/_render/glfw_renderer.py#L28 Seems like having glfw.init() run globally is the cause.