Make Gym interface work with grayscale and RGB visual observations (#2192)

gym-unity/gym_unity/envs/unity_env.py

     def _single_step(self, info):
         if self.use_visual:
             visual_obs = info.visual_observations
-            if isinstance(visual_obs, list):
-                visual_obs = np.array(visual_obs)
-                    visual_obs_list.append(self._preprocess_single(obs[0, :, :, :]))
+                    visual_obs_list.append(self._preprocess_single(obs[0]))
-                self.visual_obs = self._preprocess_single(visual_obs[0][0, :, :, :])
+                self.visual_obs = self._preprocess_single(visual_obs[0][0])
 
             default_observation = self.visual_obs
         else: