mujoco.FatalError in Ubuntu 18.04.5

when i run memory_maze i met a Error below:
`/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/glfw/__init__.py:912: GLFWError: (65544) b'X11: The DISPLAY environment variable is missing'
warnings.warn(message, GLFWError)
/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/glfw/__init__.py:912: GLFWError: (65537) b'The GLFW library is not initialized'
warnings.warn(message, GLFWError)
Traceback (most recent call last):
File "/home/DQC/demo/test.py", line 4, in
env = gym.make('dmc_memory_maze:MemoryMaze-9x9-v0')
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/gym/envs/registration.py", line 640, in make
env = env_creator(**_kwargs)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dmc_memory_maze-0.2.0-py3.7.egg/dmc_memory_maze/__init__.py", line 17, in _make_gym_env
dmenv = dm_task(**kwargs)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dmc_memory_maze-0.2.0-py3.7.egg/dmc_memory_maze/tasks.py", line 25, in memory_maze_9x9
return _memory_maze(9, 3, 250, **kwargs)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dmc_memory_maze-0.2.0-py3.7.egg/dmc_memory_maze/tasks.py", line 105, in _memory_maze
strip_singleton_obs_buffer_dim=True)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/composer/environment.py", line 331, in __init__
delayed_observation_padding=delayed_observation_padding)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/composer/environment.py", line 221, in __init__
self._recompile_physics_and_update_observables()
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/composer/environment.py", line 243, in _recompile_physics_and_update_observables
self._observation_updater.reset(self._physics_proxy, self._random_state)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/composer/observation/updater.py", line 165, in reset
enabled.observation_callable())
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/composer/observation/observable/mjcf.py", line 273, in get_observation
render_flag_overrides=self._render_flag_overrides)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/mujoco/engine.py", line 220, in render
scene_callback=scene_callback)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/mujoco/engine.py", line 694, in __init__
if self._physics.contexts.mujoco is not None:
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/mujoco/engine.py", line 525, in contexts
self._make_rendering_contexts()
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/mujoco/engine.py", line 511, in _make_rendering_contexts
mujoco_context = wrapper.MjrContext(self.model, render_context)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/mujoco/wrapper/core.py", line 606, in __init__
ptr = ctx.call(mujoco.MjrContext, model.ptr, font_scale)
File "/home/DQC/anaconda3/envs/test/lib/python3.7/site-packages/dm_control/_render/executor/render_executor.py", line 136, in call
return func(*args, **kwargs)
mujoco.FatalError: an OpenGL platform library has not been loaded into this process, this most likely means that a valid OpenGL context has not been created before mjr_makeContext was called`

首先复现 dmc_memory_maze/tasks.py 中 memory_maze 环境的创建，并记录 Ubuntu、DISPLAY、GLFW 和 MuJoCo 的设置。通过 dm_control/mujoco/engine.py 和 dm_control/_render/executor/render_executor.py 跟踪渲染上下文的创建过程。完成的标准是理解已报告的 OpenGL 初始化失败，并且在有文档记录的设置下环境创建或渲染成功。

由索引模型根据 Issue 内容生成。