Created Unassigned: MagickFormat.Bgra fails to read alpha [1344]

Test case:

```
var bytes = new byte[] { 1, 2, 3, 4 };

var imgRgba = new MagickImage(bytes,
new MagickReadSettings { Width = 1, Height = 1, Format = MagickFormat.Rgba });
var pixelRgba = imgRgba.GetReadOnlyPixels().GetPixel(0, 0);
// is 1,2,3,4 as expected

var imgBgra = new MagickImage(bytes,
new MagickReadSettings { Width = 1, Height = 1, Format = MagickFormat.Bgra });
var pixelBgra = imgBgra.GetReadOnlyPixels().GetPixel(0, 0);
// is 3, 2, 1, 255 - expected 3, 2, 1, 4

```