message: fix pygame_viewer drawWorld when no object is drawn
diff: @@ -32,16 +32,15 @@ class PygameViewer(BaseViewer):
  # NOTE: optimize to support alpha value in pygame
  # https://stackoverflow.com/questions/6339057/draw-a-transparent-rectangle-in-pygame
  - self.alpha_surf = None
  -
  + self.alpha_surf = dict()
  self.background_color = (255, 255, 255)
  pg.font.init()
  -
  try:
  self.screen = pg.display.set_mode(
  self.screen_dims, pg.DOUBLEBUF | pg.HWSURFACE)
  pg.display.set_caption("Bark")
  + self.clear()
  except pg.error:
  self.screen = None
  print("No available video device")
  @@ -205,6 +204,7 @@ class PygameViewer(BaseViewer):
  def drawWorld(self, world, eval_agent_ids=None, show=True):
  super(PygameViewer, self).drawWorld(world, eval_agent_ids)
  + if self.alpha_surf:
  for s in self.alpha_surf.values():
  self.screen_surface.blit(s, (0, 0))
  