fix "Could not create GL context" (#1889)

* fix "Could not create GL context"

* get available display in case of None spec - create context all the time

gym/envs/classic_control/rendering.py

@@ -43,19 +43,31 @@ def get_display(spec):
     Pyglet only supports multiple Displays on Linux.
     """
     if spec is None:
-        return None
+        return pyglet.canvas.get_display()
+        # returns already available pyglet_display,
+        # if there is no pyglet display available then it creates one
     elif isinstance(spec, str):
         return pyglet.canvas.Display(spec)
     else:
         raise error.Error('Invalid display specification: {}. (Must be a string like :0 or None.)'.format(spec))
 
+def get_window(width, height, display):
+    """
+    Will create a pyglet window from the display specification provided.
+    """
+    screen = display.get_screens() #available screens
+    config = screen[0].get_best_config() #selecting the first screen
+    context = config.create_context(None) #create GL context
+
+    return pyglet.window.Window(width=width, height=height, display=display, config=config, context=context)
+
 class Viewer(object):
     def __init__(self, width, height, display=None):
         display = get_display(display)
 
         self.width = width
         self.height = height
-        self.window = pyglet.window.Window(width=width, height=height, display=display)
+        self.window = get_window(width=width, height=height, display=display)
         self.window.on_close = self.window_closed_by_user
         self.isopen = True
         self.geoms = []