Add support for Decoding BI_ALPHABITFIELDS (SixLabors#832)

* Adds support for BI_ALPHABITFIELDS

* Fix for decoding bitmaps with a less than full sized palette

Author: brianpopow

tests/ImageSharp.Tests/TestImages.cs

@@ -221,6 +221,7 @@ public static class Bmp
             public const string Bit8Palette4 = "Bmp/pal8-0.bmp";
             public const string Os2v2Short = "Bmp/pal8os2v2-16.bmp";
             public const string Os2v2 = "Bmp/pal8os2v2.bmp";
+            public const string LessThanFullSizedPalette = "Bmp/pal8os2sp.bmp";
             public const string Pal8Offset = "Bmp/pal8offs.bmp";
 
             // Bitmap images with compression type BITFIELDS
@@ -232,6 +233,7 @@ public static class Bmp
             public const string Issue735 = "Bmp/issue735.bmp";
             public const string Rgba32bf56 = "Bmp/rgba32h56.bmp";
             public const string Rgba321010102 = "Bmp/rgba32-1010102.bmp";
+            public const string RgbaAlphaBitfields = "Bmp/rgba32abf.bmp";
 
             public static readonly string[] BitFields 
             = {