Fix rendering for GatherEnv

[ghost]

The following error is thrown by the code when trying to render a GatherEnv:

Creating window glfw
ERROR: GLEW initalization error: Missing GL version

The issue occurs when the render method in GatherEnv is called for the second time, and more specifically, when the MjRenderContext objects are instantiated for the second time with the same MjSim objects.
If these MjRenderContext objects are instantiated once, the problem no longer appears, but the rendering does not work properly, and a black background appears instead in the viewer window.
Therefore, this issue may require a more complete update of the render method in GatherEnv.

Note:
Currently the render method in GatherEnv is bypassed for testing purposes.

[ghost]

Also, when solving this issue, check that the implementation works for both CPU and GPU.

[jonashen]

@gonzaiva Is this issue fixed?

[ghost]

No.