1.0.4: Accept the same values of `MUJOCO_GL` as the `mujoco` package.

The `mujoco` package also makes use of the `MUJOCO_GL` environment variable to switch between rendering backends, which are implemented separately from the ones currently used in `dm_control`. However, `mujoco` accepts a wider range of `MUJOCO_GL` values, in particular it allows `off` to be specified to disable rendering entirely.

Before this change, specifying `MUJOCO_GL=off` will result in `dm_control` complaining that it is an invalid value. This change eliminates the conflicting `MUJOCO_GL` values expected by the two libraries.

Fixes #324.

PiperOrigin-RevId: 461577872
Change-Id: Ie962040395b766affc8342ab9d9fd09a0afe15cb