Add support for encoding 16 bit per pixel bitmaps (SixLabors#899)

* Implemented encoding of 16 bits per pixel bitmaps

* Add unit tests for 16 bit encoding and Bgra5551 conversion

* Add additional Bgra5551 pixel conversion tests

* Add Bgra5551 tests for Short2/4 and HalfVector2/4

* Use scaled vector conversion

tests/ImageSharp.Tests/TestImages.cs

@@ -241,6 +241,7 @@ public static class Bmp
             public const string Bit16Inverted = "Bmp/test16-inverted.bmp";
             public const string Bit32Rgb = "Bmp/rgb32.bmp";
             public const string Bit32Rgba = "Bmp/rgba32.bmp";
+            public const string Rgb16 = "Bmp/rgb16.bmp";
 
             // Note: This format can be called OS/2 BMPv1, or Windows BMPv2
             public const string WinBmpv2 = "Bmp/pal8os2v1_winv2.bmp";