title: set_clip_path on a matplotlib clabel not clipping properly
body: <p>I'm making a contour plot that is clipped to a polygon path:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  from matplotlib.patches import Polygon
  import numpy as np
  
  fig = plt.figure()
  axes = plt.subplot()
  x,y = np.meshgrid( np.linspace(-10,10,51), np.linspace(-10,10,51) ) 
  z = np.sin(np.sqrt(x**2+y**2))
  CS =  axes.contour(x, y, z, np.linspace(-1,1,11) )
  axes.set_aspect('equal')
  
  # clip contours by polygon
  radius = 8
  t = np.linspace(0,2*np.pi,101)
  x_bound,y_bound = radius*np.sin(t),radius*(np.cos(t)+0.1*(np.cos(7*t)))
  clip_map = Polygon(list(zip(x_bound,y_bound)),fc='#EEEEEE',ec='none')
  axes.add_patch(clip_map)
  for collection in CS.collections:
      collection.set_clip_path(clip_map)
  
  # label the contours    
  CLB = axes.clabel(CS, colors='black')
  for text_object in CLB:
      text_object.set_clip_path(clip_map) # Doesn't do anything!
  
  plt.show()
  </code></pre>
  
  <p>To my surprise, the labels aren't clipped despite the <code>Text</code> objects having a <code>set_clip_path</code> method that doesn't return an error:  </p>
  
  <p><a href="https://i.stack.imgur.com/K2wMd.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/K2wMd.png" alt="contour plot where labels aren&#39;t clipped"></a></p>
  
  <p>How can I clip the labels outside of the gray polygon area? Do I need to resort to manually finding the X and Y positions, calculating point in polygon, and <code>set_visible = False</code> for each <code>Text</code> item? Why doesn't this code work as-is? I'm using matplotlib version 1.5.1 and python 3.5.1.</p>