title: No transparency in Bitmap when loading from MemoryStream
body: <p>When I am writing a <code>Bitmap</code> to a file and read from the file, I am getting the transparency correctly.</p>
  
  <pre class="lang-cs prettyprint-override"><code>using (Bitmap bmp = new Bitmap(2, 2))
  {
      Color col = Color.FromArgb(1, 2, 3, 4);
      bmp.SetPixel(0, 0, col);
      bmp.Save("J.bmp");
  }
  
  using (Bitmap bmp = new Bitmap("J.bmp"))
  {
      Color col = bmp.GetPixel(0, 0);
      // Value of col.A = 1. This is right.
  }
  </code></pre>
  
  <p>But if I write the <code>Bitmap</code> to a <code>MemoryStream</code> and read from that <code>MemoryStream</code>, the transparency has been removed. All alpha values become <code>255</code>.</p>
  
  <pre class="lang-cs prettyprint-override"><code>MemoryStream ms = new MemoryStream();
  using (Bitmap bmp = new Bitmap(2, 2))
  {
      Color col = Color.FromArgb(1, 2, 3, 4);
      bmp.SetPixel(0, 0, col);
      bmp.Save(ms, ImageFormat.Bmp);
  }
  
  using (Bitmap bmp = new Bitmap(ms))
  {
      Color col = bmp.GetPixel(0, 0);
      // Value of col.A = 255. Why? I am expecting 1 here.
  }
  </code></pre>
  
  <p>I wish to save the <code>Bitmap</code> to a <code>MemoryStream</code> and read it back with transparency. How can I resolve this issue?</p>