Add a feature to generate a white noise background as init world

gte/world.py

                 self._val = np.zeros([self._y, self._x])
             else:
                 self._val = np.zeros([self._y, self._x, 3])
+        elif color == 'rand':
+            if self._gray:
+                self._val = np.random.randint(255, size=(self._y, self._x))
+            else:
+                self._val = np.random.randint(255, size=(self._y, self._x, 3))
         else:
             self._val = [[color for _ in range(self._x)] for _ in range(self._y)]
             self._val = np.array(self._val)