This PR adds a new `wait_for_textures` option in DirectRLEnv and
ManagerBasedEnv cfg classes to wait for texture loading to complete
before proceeding with rendering. Previously, textures may continue to
load while the environment is running, resulting in renders with
incomplete textures.

- Bug fix (non-breaking change which fixes an issue)

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there