title: CGBitmapContextCreate : unsupported parameter combination
body: <p>I am trying to create a 8-bit grayscale context as follows :</p>
  
  <pre><code>CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
  CGContextRef context = CGBitmapContextCreate(
      data, m_width, m_height, 8, m_width, colorSpace, 
      kCGBitmapByteOrder32Little|kCGImageAlphaNone);
  </code></pre>
  
  <p>But I have the following error :</p>
  
  <pre><code>CGBitmapContextCreate: unsupported parameter combination: 
  8 integer bits/component; 8 bits/pixel; 1-component color space; 
  kCGImageAlphaNone; 1936 bytes/row.
  </code></pre>
  
  <p>Why is this combination unsupported ?</p>