Bitmap decoder now can decode bitmap arrays (SixLabors#930)

* Bitmap Decoder can now decode BitmapArray

* Add tests for bitmap metadata decoing. Fix an issue that a bitmap with a v5 header would be set in the metadata as an v4 header.

* Fixed issue with decoding bitmap arrays: color map size was not determined correctly. Added more test images.

* Refactor colormap size duplicate declaration.

* Fixed an issue, that when an unsupported bitmap is loaded the typ marker was not correctly shown in the error message

Author: brianpopow

tests/ImageSharp.Tests/TestImages.cs

@@ -261,6 +261,14 @@ 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 Os2BitmapArray9s = "Bmp/9S.BMP";
+            public const string Os2BitmapArrayDiamond = "Bmp/DIAMOND.BMP";
+            public const string Os2BitmapArrayMarble = "Bmp/GMARBLE.BMP";
+            public const string Os2BitmapArraySkater = "Bmp/SKATER.BMP";
+            public const string Os2BitmapArraySpade = "Bmp/SPADE.BMP";
+            public const string Os2BitmapArraySunflower = "Bmp/SUNFLOW.BMP";
+            public const string Os2BitmapArrayWarpd = "Bmp/WARPD.BMP";
+            public const string Os2BitmapArrayPines = "Bmp/PINES.BMP";
             public const string LessThanFullSizedPalette = "Bmp/pal8os2sp.bmp";
             public const string Pal8Offset = "Bmp/pal8offs.bmp";
             public const string OversizedPalette = "Bmp/pal8oversizepal.bmp";