title: Pillow convert png to 8bit bitmap
body: <p>i tried to convert 8bit PNGs to 8bit(256indexed palette) Bitmap image,
  but Pillow is keep vomiting crappy outcome.</p>
  
  <p>this is what i tried.</p>
  
  <pre><code>image = Image.open(file)
  image = image.convert('P')
  pp = image.getpalette()
  pp[0] = 255
  pp[1] = 0
  pp[2] = 255
  image.putpalette(pp)
  </code></pre>
  
  <p>or</p>
  
  <pre><code>image = Image.open(file)
  image = image.convert('P')
  image.save(blabla.bmp)
  </code></pre>
  
  <p>and this is the outcome what i expected to see.
  this is an actual bitmap(done by Photoshop.)
  <a href="https://i.stack.imgur.com/LVLmd.png" rel="nofollow noreferrer">Photoshop</a>
  and this is what Pillow did:
  <a href="https://i.stack.imgur.com/p6THU.png" rel="nofollow noreferrer">Pillow</a>
  what kind of joke is this ?!
  and it even got cropped out
  what should i do to convert it correctly?</p>
  
  <p>Original Image:</p>
  
  <p><a href="https://i.stack.imgur.com/4DN3u.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4DN3u.png" alt="enter image description here"></a></p>