Colab: OpenGL.raw.EGL._errors

When I tried to run the Colab Notebook, the following error occurred at 2-v-2 Boxhead soccer.

```
Exception ignored in:
Traceback (most recent call last):
File "/usr/local/lib/python3.9/dist-packages/dm_control/mujoco/wrapper/core.py", line 633, in __del__
File "/usr/local/lib/python3.9/dist-packages/dm_control/mujoco/wrapper/core.py", line 624, in free
File "/usr/lib/python3.9/contextlib.py", line 119, in __enter__
File "/usr/local/lib/python3.9/dist-packages/dm_control/_render/base.py", line 135, in make_current
RuntimeError: Cannot make context current on thread : this context is already current on another thread <_MainThread(MainThread, started 139896762632000)>.
Exception ignored in:
Traceback (most recent call last):
File "/usr/local/lib/python3.9/dist-packages/dm_control/_render/base.py", line 118, in __del__
File "/usr/local/lib/python3.9/dist-packages/dm_control/_render/base.py", line 115, in _free_unconditionally
File "/usr/local/lib/python3.9/dist-packages/dm_control/_render/executor/render_executor.py", line 144, in terminate
File "/usr/local/lib/python3.9/dist-packages/dm_control/_render/base.py", line 96, in _free_on_executor_thread
File "/usr/local/lib/python3.9/dist-packages/dm_control/_render/pyopengl/egl_renderer.py", line 125, in _platform_make_current
File "/usr/local/lib/python3.9/dist-packages/OpenGL/error.py", line 230, in glCheckError
OpenGL.raw.EGL._errors.EGLError: EGLError(
err = EGL_BAD_ACCESS,
baseOperation = eglMakeCurrent,
cArguments = (
,
,
,
,
),
result = 0
)
```

Please let me know how to fix it.

在 2-v-2 Boxhead 足球示例的 Colab Notebook 中重现该错误。先从 dm_control/mujoco/wrapper/core.py 以及 traceback 中显示的 dm_control/_render/base.py、render_executor.py 和 pyopengl/egl_renderer.py 中的渲染路径开始。完成的标准是 notebook 不再报告 thread-conflict 和 EGL_BAD_ACCESS 清理错误。

由索引模型根据 Issue 内容生成。