title: Changed behaviour of ImageEnhance.Brightness in Pillow
body: <p>I'm trying to paste image B over image A with half opacity (i.e. pasted image is half transparent). </p>
  
  <p>In version 2.1.0 of pillow the following code worked, in version 3.3.1 it no longer works:</p>
  
  <pre><code>A = Image.open('A.png')
  
  B = Image.open('B.png')
  enhancer = ImageEnhance.Brightness(B)
  mask = enhancer.enhance(0.5)
  print(mask.getpixel((10,10)), mask.getpixel((30,30)))
  mask.save('Mask.png')
  
  A.paste(B, (0,0), mask)
  A.save('Result.png')
  </code></pre>
  
  <p>Image A is a black 'A' on a white background</p>
  
  <p>Image B is a red 'B' on a transparent background</p>
  
  <p>Images are provided below</p>
  
  <p>Version 2.1.0 produces (127,0,0,127) for pixel 30,30 of the mask</p>
  
  <p>Version 3.3.1 produces (127,0,0,255) for pixel 30,30 of the mask</p>
  
  <p><a href="https://i.stack.imgur.com/J9yVz.png" rel="nofollow noreferrer">Image A</a>
  <a href="https://i.stack.imgur.com/z1BZE.png" rel="nofollow noreferrer">Image B</a></p>